beaglebone: add kernel image and DTs to MACHINE_ESSENTIAL_EXTRA_RDEPENDS

As reported by Nicolas Dechesne:

"Basically an oe-core (distro less) minimal image won't
boot when using beaglebone machine from meta-beagleboard since you are
expecting to have the kernel image files in /boot."

This of course assumes the image you're using drags in a packagegroup with MACHINE_ESSENTIAL_EXTRA_RDEPENDS.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>