configs/beaglebone: use the new BeagleBoard/BeagleBone kernel repository

configs/beaglebone: use the new BeagleBoard/BeagleBone kernel repository

The original source URL (http://beagleboard.org/source) now redirects to
https://elinux.org/Beagleboard:BeagleBoneBlack_Rebuilding_Software_Image
which points to an official kernel repository at GitHub.

There are updated RT and non-RT kernel source trees. Use the last non-RT
release (4.19.73-ti-r29) and the omap2plus defconfig.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>