kbuild: Provide a version of merge_into_defconfig without override warnings

kbuild: Provide a version of merge_into_defconfig without override warnings

While warning on overridden Kconfig options is a good default for merging
config fragements sometimes that is our explicit intent and the warnings
are unhelpful, add a new merge_into_defconfig_override which does the merge
but with warnings suppressed.

Since merge_into_defconfig accepts any number of fragments it is difficult
to allow it to accept the flag.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230203-arm64-defconfigs-v1-2-cd0694a05f13@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>