beaglebone: update to TI 3.12 kernel

Git repo version is taken from meta-ti.
It's a 3.12.10 kernel with TI's own patches on top.

The defconfig has been kept rather close to our old defconfig.
The kernel has been boot-tested using an NFS-root.

Apart from the kernel change, the linux headers has also been
updated from 3.8.12 to 3.12.10.

[Peter: drop UEVENT_HELPER_PATH setting]
Signed-off-by: Anders Darander <anders@chargestorm.se>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>