kconfig/merge_config.sh: Support KCONFIG_CONFIG

All make targets support $KCONFIG_CONFIG because they
run scripts/kconf.  Make sure merge_config.sh accesses the
correct file in all cases.

Previously this script broke in two different code paths,
one for targets like kvmconfig (which use merge_config.sh -m
then call a target that respects KCONFIG_CONFIG) and one for
direct use of the script without -m, which called make rules
that edit KCONFIG_CONFIG but verified a different file.

Signed-off-by: Gabriel de Perthuis <g2p.code@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>