beaglebone: update to linux 3.8

As the beaglebone support is mainlined, there's no support for the old
board setup files, and the only supported run-time configuration is through
the device tree.

We're also setting the kernel headers to follow the 3.8 series.

Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>