scripts: merge_config: add strict mode to fail upon any redefinition

When merging configuration fragments, it might be of interest to
identify mismatches (redefinitions) programmatically. Hence add the
option -s (strict mode) to instruct merge_config.sh to bail out in
case any redefinition has been detected.

With strict mode, warnings are emitted as before, but the script
terminates with rc=1. If -y is set to define "builtin having
precedence over modules", fragments are still allowed to set =m (while
the base config has =y). Strict mode will tolerate that as demotions
from =y to =m are ignored when setting -y.

Signed-off-by: Matthias Maennich <maennich@google.com>
Reviewed-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>