Add support for build on ARM-Plattform

We start supporting building the bcm-mailbox on ARM plattforms,
following plattforms are tested today:

- raspberry pi (running tinycore linux)
- bur am335x power panel (cortex a8, ti-am3352)

for compiling just give the plattform and the cross-compiler toolchain
prefix.

PLATTFORM=armv6 CROSS_COMPILE=/opt//rpi-tools/arm-bcm2708/arm-
bcm2708hardfp-linux-gnueabi/bin/arm-bcm2708hardfp-linux-gnueabi- make

will build the bcm for raspberry pi.

toolchain for armv6 (raspberry pi) can be downloaded from:
https://github.com/raspberrypi/tools

toolchain form armv7hf (bur-am335x pp) can be downloaded from:
http://releases.linaro.org/14.09/components/toolchain/binaries/gcc-linaro-arm-linux-gnueabihf-4.9-2014.09_linux.tar.xz

Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>