scripts: added Makefile to handle an external riscv32 toolchain

Compiling Zephyr with an external riscv32 toolchain would
require the following env variables to be exported:

export ZEPHYR_GCC_VARIANT=riscv32
export RISCV32_TOOLCHAIN_PATH=/PATH/TO/TOOLCHAIN/BINARY

Change-Id: I2072ed9079a4cabd27837ab39b947bc0a0a1a8b4
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>
1 file changed