ci: enable iq-x5121-evk in github CI jobs by default

Update github workflow to build iq-x5121-evk in CI jobs.

Signed-off-by: Xinzheng Long <xinzheng.long@oss.qualcomm.com>

Author: xinlon-z

.github/workflows/build-yocto.yml

@@ -142,6 +142,7 @@ jobs:
           - iq-615-evk
           - iq-8275-evk
           - iq-9075-evk
+          - iq-x5121-evk
           - iq-x7181-evk
           - kaanapali-mtp
           - qcm6490-idp