merge_config.sh: Allow to define config prefix

with CONFIG_ environment variable.

merge_config.sh uses CONFIG_ which is used in kernel and other projects.
There are some projects which use kconfig with different prefixes (e.g.
buildroot: BR2_ prefix). CONFIG_ variable is already used for this
purpose in kconfig binary (scripts/kconfig/lkc.h), let's use the same
rule for in merge_config.sh.

Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>