kbuild: arm: Do not define "comma" twice

The definition of "comma" exists in scripts/Kbuild.include.
We should not double it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Michal Marek <mmarek@suse.cz>