u-boot patches

Two changes:

* Base u-boot on the official denx repository and add gumstix patches.  I believe the current approach of having a recipe that references a branch on a gumstix fork is fragile: pushes to that branch will result in content changes to the u-boot package without any reflection of that in version number.

* Back-port an upstream feature available on most omap boards that reads environment settings from an mmc file during the boot process.  This allows image-specific environment settings to be coupled with the image, while not contaminating a clean NAND environment as desired when using the same physical board for a variety of purposes.

I recommend pushing the patches that are accepted (which should be at least the first two) upstream so they can be dropped from future gumstix BSP releases.