Refactor workflow and outboards

- change "merge" option to "branches"
 - use first branch as base and merge rest
 - use "zmk" file instead of manifest
 - checkout to subdirectories
 - checkout zmk manually
 - use west just for zephyr
 - use absolute paths
 - handle simultaneous board and shield outboards
 - change outboard and directory names

config/west.yml

@@ -1,11 +0,0 @@
-manifest:
-  remotes:
-    - name: zmkfirmware
-      url-base: https://github.com/zmkfirmware
-  projects:
-    - name: zmk
-      remote: zmkfirmware
-      revision: main
-      import: app/west.yml
-  self:
-    path: config