version: Move version_string[] from version.h to version_string.h

More C files do not use compile time timestamp macros and do not have to be
recompiled every time when SOURCE_DATE_EPOCH changes.

This patch moves version_string[] from version.h to version_string.h and
updates other C files which only needs version_string[] string to include
version_string.h instead of version.h. After applying this patch these
files are not recompiled every time when SOURCE_DATE_EPOCH changes.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Tom Rini <trini@konsulko.com>