machines: sync u-boot config name with boards.cfg

Recent u-boots (e.g. 2014.04-rc) need the _config appended to work, so
add that and sync the names with boards.cfg as well.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Merge pull request #56 from koenkooi/ubootmachine-fix

machines: sync u-boot config name with boards.cfg