[oe] Consistent MAC address for Beagleboard xM

[Matt Johnson]

Sorry about the previous 2 emails, didn't know how the text file mapped to the email.

Let me know if the code or the patch submission need to be improved.  Two potential
improvements I've thought about are checking for a BeagleBoard xM instead of just
CONFIG_MACH_OMAP_BEAGLE and making the MAC out of a cryptographically-sound hash,
rather than the raw die ID bits, in case those bits give away too much.