[U-Boot] Support of Beagleboard-xm

Hi all, I have got a Beagleboard-xm, and I'm feeling that saving environment variable is an issue because this board has no NAND.

U-Boot supports environment variable file located in a FAT file system. So I set out and made a new board omap3_beagle-xm, copied from omap3_beagle, but with environment file on the FAT of the microSD.

I'm not sure if I'm just reinventing the wheel!

Regards, Thomas