Increase the FreeBSD/arm image sizes back to 1G for 11-CURRENT.
While 480M is sufficient for 10-STABLE, 11-CURRENT images at
this size fail due to insufficient space.

This commit is solely for the sake of getting updated snapshot
builds out, after which I'll analyze the resulting images to
figure out what a more sane value is, even if the image size
for 11-CURRENT needs to differ from 10-STABLE.

Sponsored by: The FreeBSD Foundation