xtensa-build-zephyr.sh: build against SOF Zephyr stable-v2.1 branch#5519

kv2019i wants to merge 1 commit intothesofproject:stable-v2.1thesofproject/sof:stable-v2.1from

kv2019i:topic/sof-2-1-zephyr-buildkv2019i/sof:topic/sof-2-1-zephyr-buildCopy head branch name to clipboard