base-files: beagleboard: profile: use nano

base-files: beagleboard: profile: use nano

If nano is available and EDITOR isn't otherwise set, then use it.
Advanced users who would prefer vi will know how to set EDITOR anyway.
I can clearly see myself being frustrated typing 'export EDITOR=vi'
every time I start a BeagleBoard test ramdisk image, but better me have
that than to try to explain to someone fresh how to use vi.

Updated with feedback from Gary Thomas to avoid overwriting anyone
else's preference.

Signed-off-by: Jason Kridner <jkridner@beagleboard.org>
Signed-off-by: Denys Dmytriyenko <denis@denix.org>