board/beaglebone: skip bonegreen.dtb if not built

The beaglebone_qt5_defconfig uses an older kernel version which does not
provide a device tree for the beagle bone green.
post-image.sh now selects genimage_linux41.cfg if am335x-bonegreen.dtb is
not build and genimage.cfg otherwise.

Signed-off-by: Lothar Felten <lothar.felten@gmail.com>
[Thomas: minor tweaks.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>