create_disk: increase /boot partition size to 512mb

[jmarrero]

Increases the /boot partition size to 512MB as we have seen a couple of cases were kernel installs fail due to /boot size.
coreos/fedora-coreos-tracker#1247

[dustymabe]

I'm pretty sure that making this change is more disruptive than you might think. @bgilbert would know for sure.

Also we want the disks for the various architectures to stay matched up if possible: coreos/fedora-coreos-tracker#855

[miabbott]

Butane has the 384 value hardcoded, too:

$ git grep 'bootV1SizeMiB     = 384'
config/fcos/v1_3/translate.go:54:       bootV1SizeMiB     = 384
config/fcos/v1_4/translate.go:54:       bootV1SizeMiB     = 384
config/fcos/v1_5_exp/translate.go:58:   bootV1SizeMiB     = 384

[jmarrero]

I'll close this for now and we can continue the discussion on: coreos/fedora-coreos-tracker#1247