rpi-base.inc: handle empty/undefined KERNEL_DEVICETREE
rpi-base.inc: handle empty/undefined KERNEL_DEVICETREE

This patch is based on an analog patch from Christopher Boyd <xpboyd@gmail.com> committed at meta-freescale.
(see https://github.com/Freescale/meta-freescale/commit/954d7a7d7afc3ad4950eb0fd354a6f4bd06d911a)

Signed-off-by: Oliver Lang <465bfb1e9e20c68580e2738582f87dd5c4002cef@gmail.com>

agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi