merge_config.sh: rename MAKE to RUNMAKE

The variable "MAKE" is used to store the command name that has
invoked the Makefile.  (Actually, it is already set to "make"
if you run this script from a Makefile.)

In this script, however, it is used to determine if Make should be
run or not.  It is not what we usually expect.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>