beaglebone: better support for starting and creating images

- Added short readme file with description of first steps how to start.
- Added uEnv.txt so the board can start from SDCard
- Added post-image.sh script to copy included uEnv.txt to output/images

[Peter: strip trailing spaces]
Signed-off-by: Marcin Jabrzyk <marcin.jabrzyk@gmail.com>
Tested-by: Stephan Hoffmann <sho@relinux.de>
Tested-by: Marcin Jabrzyk <marcin.jabrzyk@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>