Reduce the arm/armv6 image size from 1G to 480M.

Since the images are effectively mostly zeros at 1G,
reduce the size to allow installation on smaller SD
cards, such as 512Mb.

While here, stop writing the /boot.txt file on the
WANDBOARD, which isn't used anyway.

Discussed with: imp
MFC after: 3 days
Sponsored by: The FreeBSD Foundation