kconfig: merge_config: avoid false positive matches from comment lines

The current SED_CONFIG_EXP could match to comment lines in config
fragment files, especially when CONFIG_PREFIX_ is empty. For example,
Buildroot uses empty prefixing; starting symbols with BR2_ is just
convention.

Make the sed expression more robust against false positives from
comment lines. The new sed expression matches to only valid patterns.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>