Add new target feature 'dt-overlay' which makes DTC keep the symbol
names in the generated dtb.
Make sure additional DT overlay sources specified by the new device
variable DEVICE_DTS_OVERLAY get compiled together with the main DTS
(currently overlays got to be in the same folder). Let Build/fit pass
the generated DT overlay blobs to mkits.sh.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>