Switch to MiB units for partition sizes.

Previously one had to calculate how large the /boot/firmware partition
would be, but expressing it directly in MiB units is much clearer.

This also has the benefit that the /boot/firmware partition's size would
not change if the total image size would be changed.
Such a change should be a deliberate decision and not some side-effect.
As that 'side-effect' did happen since first submitting this patch,
revert the /boot/firmware partition's size back to 300MB.

Signed-off-by: Diederik de Haas <didi.debian@cknow.org>

raspi_master.yaml

@@ -12,12 +12,12 @@ steps:
     fs-type: 'fat32'
     device: "{{ output }}"
     start: 4MiB
-    end: 20%
+    end: 300MiB
     tag: /boot
 
   - mkpart: primary
     device: "{{ output }}"
-    start: 20%
+    start: 300MiB
     end: 100%
     tag: /