release: arm - increase IMAGE_SIZE

For some reason, while 3072M is sufficient for 14-CURRENT, it is not
for 13-STABLE. Notably, previous investigations suggest that there
are changes to makefs(8) in main that do not exist in stable/13,
in which 14-CURRENT seems perfectly happy to ignore the target image
size is smaller than the data being populated to it.

I have no futher investigative details at the moment, but as this had
caused arm failures for the past three weeks, this is the more hasty
measure, hence the MFC timeframe noted.

MFC after: 10 minutes
Sponsored by: Rubicon Communications, LLC ("Netgate")