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>