[tshering]

This is how I create KoboRoot.tgz on Windows, in case it might be helpful to anybody.
Create this batch file (spoiler) on (e.g.) c:\mykobotools\, and name it, for instance, createKBRoot.bat.

Spoiler:

modify the path to the z-zip executable
SET the7zipexe="C:\Programme\7-Zip\7z.exe"

@ECHO off

SET the7zipexe="C:\Programme\7-Zip\7z.exe"

IF (%~f1) == () GOTO NothingDropped
IF NOT EXIST %~f1\NUL GOTO FileDropped


CD %~f1
REM --- clean up
IF EXIST KoboRoot.tar DEL KoboRoot.tar
IF EXIST KoboRoot.tgz DEL KoboRoot.tgz
 

REM --- make tar
"%the7zipexe%" a KoboRoot.tar -ttar  * -r  -x!_*
REM --- make tgz
"%the7zipexe%" a KoboRoot.tgz -tgzip  KoboRoot.tar

REM --- clean up
IF EXIST KoboRoot.tar DEL KoboRoot.tar

GOTO ByBy

REM ---- here starts error handling
:NothingDropped
ECHO drop folder file on this batch file!
GOTO ByBy

:FileDropped
ECHO drop folder on this batch file!
GOTO ByBy

:ByBy
PAUSE

• create the needed content in a dedicated folder, e.g.
  c:\myKoboRoot\usr\local\Kobo\libnickel.so.1.0.0
  c:\myKoboRoot\usr\local\Kobo\librmsdk.so.1.0.0
• open two windows in explorer (file manager)
  navigate the first window to c:\.
  navigate the second window to c:\mykobotools\
• drag and drop the folder myKoboRoot unto the batch file (createKBRoot.bat)
• find KoboRoot.tgz in c:\myKoboRoot\

Note: The batch file must be on the same drive as the target directory! Don't use spaces in the target path.