Enlarge firmware partition to 508MB

The firmware partition holds a copy of the initramfs and the kernel and
over the years we have seen a steady increase in its sizes.
Resizing the firmware partition later on is cumbersome as the root
partition follows directly, so it's better to make the firmware
partition not too small. A size of 508MB should be enough to accommodate
4-5 kernels+initramfs, which seems desirable.

raspi_master.yaml

@@ -12,12 +12,12 @@ steps:
     fs-type: 'fat32'
     device: "{{ output }}"
     start: 4MiB
-    end: 300MiB
+    end: 512MiB
     tag: /boot
 
   - mkpart: primary
     device: "{{ output }}"
-    start: 300MiB
+    start: 512MiB
     end: 100%
     tag: /