Add support for distro_bootcmd when using u-boot >= 2015.04+dfsg1-1 (Closes: #786367).

Do so by adjusting bootscr.beaglebone: use results from distro_bootcmd
and use previous settings as a fallback. Thanks, Vagrant Cascadian!

Signed-off-by: Cyril Brulebois <kibi@debian.org>