Extend boot partition size for RPi4

In 62a8cd04 we increased the size of the
boot partition on the RPi4 to ensure that there is enough space to store
at least two versions of the kernel. However this was only done for
standard apt images as ostree images use ostree-image-rpi64.yaml rather
than image-rpi64.yaml.

Repeat this partition size change for the OSTree images as well as we
are still seeing failing upgrade tests with the following error message:

May 19 02:13:03 apertis apertis-update-[1029]: Ostree upgrade failed: Installing kernel: regfile copy: No space left on device

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>