meta-zephyr-sdk-build.sh: Build Xtensa toolchain
meta-zephyr-sdk-build.sh: Build Xtensa toolchain

Added instructions to build Xtensa toolchain.

Change-Id: I68c5a01a5571f6bf2e95e0139d21415c2d121c56
Signed-off-by: Juro Bystricky <632519cd04f15aec8742590453c2132c1299a8ac@intel.com>

zephyrproject-rtos/meta-zephyr-sdk,zephyrproject-rtos/meta-zephyr-sdk,zephyrproject-rtos/meta-zephyr-sdk,zephyrproject-rtos/meta-zephyr-sdk