fdt: boot_get_fdt: android: use ENV 'fdtaddr' as fallback

Our platform doesn't store the DTB into the Android image second area,
but rather copies the DTB to RAM from a dedicated dtb.img partition [0],
prior to booting the Android image by calling bootm.

Similar to [1], we find it useful to just call 'bootm' and have the
right DTB being passed to OS (assuming its address has been previously
stored in 'fdtaddr' by calling `fdt addr <dtb-addr>`).

Booting Android with DTB from 'fdtaddr' will only occur if:
 - No DTB is embedded in the second area of Android image
 - 'fdtaddr' points to a valid DTB in RAM

[0] https://source.android.com/devices/architecture/dto/partitions
[1] https://patchwork.ozlabs.org/patch/1046652/


    ("Support boot Android image without address on bootm command")

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Simon Glass <sjg@chromium.org>