volla-mimameid: fix coming (back) from Android

- If the current slot is 'b' (as it can be if Android has been updated),
  even if we set the (new) active slot to 'a', we still have to
  explicitly spell out the slot.
- vbmeta has to be flashed, replacing the one from Android. Use a
  generic vbmeta from Google. In the future, the bootstrap tarball could
  include one.

v2/devices/mimameid.yml

@@ -57,6 +57,11 @@ operating_systems:
                   checksum:
                     sum: "c0216239cc6dc11e23940884d0f6d1183b29898e63b561da8122bf69636f6c7c"
                     algorithm: "sha256"
+                # TODO: tells Volla to ship a vbmeta.img as part of the bootstrap zip.
+                - url: "https://dl.google.com/developers/android/qt/images/gsi/vbmeta.img"
+                  checksum:
+                    sum: "f6da5489fd877cb69cf61fa721cfd6d77e530084aefe9b96664f818947ff61f6"
+                    algorithm: "sha256"
         condition:
           var: "bootstrap"
           value: true
@@ -87,12 +92,16 @@ operating_systems:
       - actions:
           - fastboot:flash:
               partitions:
-                - partition: "lk"
+                - partition: "lk_a"
                   file: "unpacked/lk.img"
                   group: "firmware"
-                - partition: "boot"
+                - partition: "boot_a"
                   file: "unpacked/boot.img"
                   group: "firmware"
+                - partition: "vbmeta_a"
+                  file: "vbmeta.img"
+                  group: "firmware"
+                  flags: [ "--disable-verity" ]
                 - partition: "super"
                   file: "unpacked/super.img"
                   group: "firmware"