[knc1]

Hmm...

On my K5-Paperwhite (with the libraries I thought I shipped):

[root@kindle bin]# LD_DEBUG=libs ./busybox
      5772:     find library=libc.so.6 [0]; searching
      5772:      search path= too long, not interesting:
:/mnt/us/extensions/system/bin/../usr/lib            (RUNPATH from file ./busybox)
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/tls/v7l/neon/vfp/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/tls/v7l/neon/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/tls/v7l/vfp/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/tls/v7l/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/tls/neon/vfp/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/tls/neon/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/tls/vfp/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/tls/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/v7l/neon/vfp/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/v7l/neon/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/v7l/vfp/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/v7l/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/neon/vfp/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/neon/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/vfp/libc.so.6
      5772:       trying file=/mnt/us/extensions/system/bin/../lib/libc.so.6
      5772:     
      5772:     
      5772:     calling init: /mnt/us/extensions/system/bin/../lib/libc.so.6
      5772:     
      5772:     
      5772:     calling init: /usr/lib/libenvload.so
      5772:     
      5772:     
      5772:     initialize program: ./busybox
      5772:     
      5772:     
      5772:     transferring control: ./busybox
      5772:     
BusyBox v1.22.1 (2014-06-11 02:23:01 CDT) multi-call binary.
BusyBox is copyrighted by many authors between 1998-2012.
Licensed under GPLv2. See source distribution for detailed
copyright notices.

Usage: busybox [function [arguments]...]
   or: busybox --list[-full]
   or: busybox --install [-s] [DIR]
   or: function [arguments]...

        BusyBox is a multi-call binary that combines many common Unix
        utilities into a single executable.  Most people will create a
        link to busybox for each function they wish to use and BusyBox
        will act like whatever it was invoked as.

Currently defined functions:
        [, [[, addgroup, adduser, ar, arping, ash, awk, basename, blkid, bunzip2, bzcat, cat, catv, chattr, chgrp, chmod,
        chown, chroot, chrt, chvt, cksum, clear, cmp, cp, cpio, crond, crontab, cut, date, dc, dd, deallocvt, delgroup,
        deluser, devmem, df, diff, dirname, dmesg, dnsd, dnsdomainname, dos2unix, du, dumpkmap, echo, egrep, eject, env,
        ether-wake, expr, false, fdflush, fdformat, fgrep, find, fold, free, freeramdisk, fsck, fstrim, fuser, getopt, getty,
        grep, gunzip, gzip, halt, hdparm, head, hexdump, hostid, hostname, hwclock, id, ifconfig, ifdown, ifup, inetd, init,
        insmod, install, ip, ipaddr, ipcrm, ipcs, iplink, iproute, iprule, iptunnel, kill, killall, killall5, klogd, last,
        less, linux32, linux64, linuxrc, ln, loadfont, loadkmap, logger, login, logname, losetup, ls, lsattr, lsmod, lsof,
        lspci, lsusb, lzcat, lzma, makedevs, md5sum, mdev, mesg, microcom, mkdir, mkfifo, mknod, mkswap, mktemp, modprobe,
        more, mount, mountpoint, mt, mv, nameif, netstat, nice, nohup, nslookup, od, openvt, passwd, patch, pidof, ping,
        pipe_progress, pivot_root, poweroff, printenv, printf, ps, pwd, rdate, readlink, readprofile, realpath, reboot, renice,
        reset, resize, rm, rmdir, rmmod, route, run-parts, runlevel, sed, seq, setarch, setconsole, setkeycodes, setlogcons,
        setserial, setsid, sh, sha1sum, sha256sum, sha3sum, sha512sum, sleep, sort, start-stop-daemon, strings, stty, su,
        sulogin, swapoff, swapon, switch_root, sync, sysctl, syslogd, tail, tar, tee, telnet, test, tftp, time, top, touch, tr,
        traceroute, true, tty, udhcpc, umount, uname, uniq, unix2dos, unlzma, unxz, unzip, uptime, usleep, uudecode, uuencode,
        vconfig, vi, vlock, watch, watchdog, wc, wget, which, who, whoami, xargs, xz, xzcat, yes, zcat

      5772:     
      5772:     calling fini: ./busybox [0]
      5772:     
      5772:     
      5772:     calling fini: /usr/lib/libenvload.so [0]
      5772:     
[root@kindle bin]#

Note: There is no "grep -v" in the shipped package -
Your trace shows it not found in the hard-float package and is then searching for the soft-float version (which should have worked - but I didn't test it).

The only thing in the shipped package in */bin is busybox (one (1) file).

Aw, so!
Did you: rm -fr /mnt/us/extensions/system before you copied in the new hard-float stuff? (mine was empty)
There may be a conflict with what you already had in there.

Or with alias'es set-up in your root environment (mine had none).
Do (still) have root's profile set-up with an alias grep="something evil" ??