u-boot: base on upstream repository

Standard practice in OE has been to base u-boot on the official denx
repository with patches specific to the platform.  Using a gumstix forked
repository and a branch for which the relevant patches are not reflected in
a PR number or any other indicator leads to confusion and the risk of
incompatible packages if that branch is updated.

Revert to using the official repository with a specific base version
(corresponding to tag v2013.07) and specific patches, which should in turn
be pushed upstream so they can be removed from recipes based on later u-boot
releases.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
u-boot: read boot environment overrides from mmc card

Image-specific environment settings (such as command-line options related to
DSP features) can cause boot problems when mis-applied to images that don't
support them, as happens when the settings are stored on the overo COM
itself.  Back-port a u-boot enhancement (available on most OMAP platforms)
that reads uEnv.txt from the mmc card prior to booting.  This allows
features settings required for certain images used on a particular Gumstix
device to be coupled with the SD card used to boot that image.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Merge pull request #4 from pabigot/for-dylan/u-boot

u-boot patches.  Seems reasonable---thank you.