doc: Add environment variables to getting_started.rst

Add GCCARMEMB_TOOLCHAIN_PATH and ZEPHYR_GCC_VARIANT to
getting_started.rst per ZEP-1027

Change-Id: Ia73697c66562e02a6d93afdac6cc92e327189abe
Signed-off-by: Evan Couzens <evanx.couzens@intel.com>
1 file changed