kbuild: reduce output spam when building out of tree

kbuild: reduce output spam when building out of tree

The execution of $(call cmd,makefile) will print 'GEN Makefile' on each
build, even if the Makefile is not effectively changed.

Use a filechk command instead, so a message is only printed on changes.

The Makefile is now created even if the build is aborted due to an
unclean working tree. That should not make a difference in practice.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260305-kbuild-makefile-spam-v1-1-910f6cf218a1@linutronix.de
Signed-off-by: Nicolas Schier <nsc@kernel.org>