dev-tree: Support non-skales DTB if only one appended

If people append a unique DTB which has not been generated
with skales/dtbTool, we consider they expect to use it
without any compatibility check/filtering.

You can generate such boot.img with generic android mkbootimg tool:
$ gzip -c Image > Image.gz
$ cat Image.gz Image.dtb > Image.gz+dtb
$ mkbootimg --kernel ./Image.gz+dtb --ramdisk ./initrd.img \
  --output boot.img --pagesize 2048 --base 80000000
  --cmdline "root=/dev/mmcblk0p10 rw rootwait console=ttyMSM0,115200n8"

Change-Id: I2334b50136dcc6094fd1e58563a38f03542eadaa
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>