Adding cross-compile for Erlang on the LEGO EV3.

When configuring, the EV3_SYSROOT environment var should be set to the "extra/linux-devkit/arm-none-linux-gnueabi" sub-dir of the EV3 toolchain (https://github.com/mindboards/ev3sources.git).  This also expects the appropriate CodeSourcery ARM cross-compile toolchain to be present and working.