CI: don't use debug-tweaks

As of the following commit in oe-core[1]:

  classes-recipe/core-image: drop debug-tweaks IMAGE_FEATURE

The debug-tweaks feature is no longer valid. Replace it with the options
that we need to perform login over testimage.

[1] https://git.openembedded.org/openembedded-core/commit/?id=2c229f9542c6ba608912e14c9c3f783c3fa89349

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/debug.yml

@@ -5,5 +5,5 @@ header:
 
 # Add universally helpful features when testing boards
 local_conf_header:
-  debug: |
-    EXTRA_IMAGE_FEATURES:append = " debug-tweaks"
+  rootlogin: |
+    EXTRA_IMAGE_FEATURES:append = " allow-empty-password empty-root-password allow-root-login"