MAKEALL: fix a bug to use CROSS_COMPILE_<ARCH>

Commit 27af930e changed the boards.cfg format but
missed to change get_target_arch() fuction.
This commit adjusts it for CROSS_COMPILE_<ARCH>
to work correctly.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>