busybox: build arch specific binaries

BusyBox was building "arm" binaries for all variants.

Instead, build using BUSYBOX_ARCH, which defaults to be the same arch as
the kernel. BUSYBOX_ARCH can be over-ridden in the platform config, eg.
to specifically use an ARM/AARCH32 busybox on an AARCH64 system.

Change-Id: Ife7bb26d618c8d7764f84e356e79f21e0eb502e2
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>