build: use 128MiB as the boot/kernel partition size on armvirt target

build: use 128MiB as the boot/kernel partition size on armvirt target

The nominal partition type for EFI boot partitions is FAT32,
which has a minimum size of 32MiB on a 512-byte-sector block device.

To ensure that the boot partition is created as FAT32 set a size
well above this minimum.

A useful discussion about EFI partition sizes can be found here:
https://superuser.com/questions/1310927/what-is-the-absolute-minimum-size-a-uefi-system-partition-can-be

I have found 128MiB works pretty consistently across both
tools (mkfs.fat) and firmwares (EDKII)

Signed-off-by: Mathew McBride <matt@traverse.com.au>