scripts: merge_config: Add option to suppress warning on overrides

Currently merge_config.sh will unconditionally warn if a fragment overrides
any already set symbol. This is generally desirable but is inconvenient in
cases where we want to create a fragment which disables unwanted options in
the base configuration, for example when attempting to produce a smaller
version of another configuration.

Add an option -Q which will suppress these warnings.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230203-arm64-defconfigs-v1-1-cd0694a05f13@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>