Makefile: fix build timestamps ...

This patch adds the scripts/mkcompile_h from the Kernel. This gives
us additional information about the build like who built this binary
and with which compiler. Currently the only information used is the
build timestamp and the generation number.
This also fixes the build timestamp in the banner which is now correct
even with consecutive builds without making clean in between.

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