I'm guessing you need u-boot-tools on your host

.
Code:
equery f u-boot-tools
* Searching for u-boot-tools ...
* Contents of dev-embedded/u-boot-tools-2015.04:
/etc
/etc/fw_env.config
/usr
/usr/bin
/usr/bin/bmp_logo
/usr/bin/easylogo
/usr/bin/fw_printenv
/usr/bin/fw_setenv -> fw_printenv
/usr/bin/gen_eth_addr
/usr/bin/img2srec
/usr/bin/mkimage
/usr/lib
/usr/lib/debug
/usr/lib/debug/usr
/usr/lib/debug/usr/bin
/usr/lib/debug/usr/bin/bmp_logo.debug
/usr/lib/debug/usr/bin/easylogo.debug
/usr/lib/debug/usr/bin/fw_printenv.debug
/usr/lib/debug/usr/bin/gen_eth_addr.debug
/usr/lib/debug/usr/bin/img2srec.debug
/usr/lib/debug/usr/bin/mkimage.debug
/usr/share
/usr/share/man
/usr/share/man/man1
/usr/share/man/man1/mkimage.1