Make: update setlocalversion from the kernel ...

This syncs the localversion generation with the kernel.

At least one effect is that a barebox built from a tar archive no
longer marks itself as dirty. Also it moves some stuff from a scary
Makefile to a not-so-scary shell script.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Build system: Use setlocalversion from kernel ...

Autogenerate a local version from git.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>