FP4: Add bootloader unlock check & set_active for postmarketOS

Similar to the Ubuntu Touch installation steps, for postmarketOS the
bootloader needs to be unlocked. Also making sure we use the _a
partition can't be a bad idea.

v2/devices/FP4.yml

@@ -217,12 +217,28 @@ operating_systems:
           postmarketos:interfaces:
     prerequisites: []
     steps:
+      ### Ensure we always start in bootloader mode
       - actions:
           - adb:reboot:
               to_state: "bootloader"
         fallback:
           - core:user_action:
               action: "bootloader"
+      ### Ensure the bootloader has been unlocked already
+      - actions:
+          - fastboot:wait:
+      - actions:
+          - fastboot:assert_var:
+              variable: "unlocked"
+              value: "yes"
+        fallback:
+          - core:user_action:
+              action: "unlock_phone"
+      ### As this is an A/B device, force all future operations in "a" slot.
+      - actions:
+          - fastboot:set_active:
+              slot: "a"
+      ### Flash partitions
       - actions:
           - postmarketos:download:
           - fastboot:erase: