Fix various issues to build core-image-minimal for all jetson variant

[aragua]

This patch add several fixes to make rauc work on jetson:

• add setexpr command in u-boot to allow boot count
• fix bootcommand (set replaced by setenv)
• Divide partition size by 2 in core-image-minimal to avoid flash issue
• Use ${MACHINE} as compatible machine in system.conf and update-bundle

[aragua]

I've splitted commits and change the signed off but can't update that PR So i will close it and open a new one.

[ejoerns]

@aragua You should have been able to update this again by force-pushing your branch.

Thanks for splitting up and fixing the Signed-off-by! :+1: