jforissier documentation: clarify $(ARCH)
$(ARCH) has to be 'arm' for both 32-bit and 64-bit Arm builds.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Latest commit b8ea856 Aug 30, 2018