MFC r284333:

MFC r284333:
  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.

Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@284517 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f