kbuild: Allow to override LINUX_COMPILE_BY and LINUX_COMPILE_HOST macros

Make it possible to override the user@host string displayed during boot
and in /proc/version by the environment variables KBUILD_BUILD_USER and
KBUILD_BUILD_HOST. Several distributions patch scripts/mkcompile_h to
achieve this, so let's provide an official way. Also, document the
KBUILD_BUILD_TIMESTAMP variable while at it.

Signed-off-by: Michal Marek <mmarek@suse.cz>