merge_config.sh: Check error codes from make

When we execute make after merging the configurations we ignore any
errors it produces causing whatever is running merge_config.sh to be
unaware of any failures.  This issue was noticed by Guillaume Tucker
while looking at problems with testing of clang only builds in KernelCI
which caused Kbuild to be unable to find a working host compiler.

This implementation was suggested by Yamada-san.

Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>