Blackfin: update tiny board settings (voltage/default env)

The tinyboards like to run at a little lower voltage than the default, and
they prefer to boot over the network.  For the latter, extend the common
code a little to make this easier.

Also fix the cm-bf527 env sector size while we're in here to reflect the
flash that is actually in use.

Signed-off-by: Harald Krapfenbauer <Harald.Krapfenbauer@bluetechnix.at>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>