ci/base: fix EXTRA_IMAGE_FEATURES assignment

Use '+=' to avoid rewriting and deleting previously written content.

Signed-off-by: Jose Quaresma <jose.quaresma@oss.qualcomm.com>

Author: quaresmajose

ci/base.yml

@@ -52,7 +52,7 @@ local_conf_header:
     IMAGE_FSTYPES += "qcomflash"
   extra: |
     DISTRO_FEATURES:append = " efi pam pni-names"
-    EXTRA_IMAGE_FEATURES = "allow-empty-password empty-root-password allow-root-login"
+    EXTRA_IMAGE_FEATURES += "allow-empty-password empty-root-password allow-root-login"
     IMAGE_ROOTFS_EXTRA_SPACE = "307200"
     WATCHDOG_RUNTIME_SEC:pn-systemd = "30"