beaglebone: update to latest U-Boot

Update to the default version of U-Boot, at this time 2013.10.

Tested with an old uEnv.txt (i.e. using uImage) and with the
currently prefered zImage.

Signed-off-by: Anders Darander <anders@chargestorm.se>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>