build: arm: Make use of CROSS_COMPILE for build

This enables the build scripts to use a toolchain selected by the
CROSS_COMPILE environment variable, which is commonly used for Linux cross
compiling tasks. The previously used OE_BASE variable is dropped as it can be
replaced with a proper CROSS_COMPILE setting easily.

Change-Id: Icde46384458bdf798efa5965ca51fd9adec48a15
Signed-off-by: Julian Scheel <julian@jusst.de>