libfdt: overlay_merge: Introduce fdt_overlay_merge()

fdt_overlay_merge() merges two overlay blobs. This is largely expected to be
used offline on a build machine to combine two or more overlay blobs into one.

This is intended to help maintain device-tree overlay code in
multiple source repositories, but merge their binary forms (overlay blobs)
into one so that bootloader's task of searching for all relevant overlay blobs
is simplified.

This patch introduces fdt_overlay_merge() which is exactly identical to
fdt_overlay_apply(). Subsequent patches will introduce required changes to merge
overlay blobs.

Change-Id: I2d278227f53cb08e7c5b1e90b15062a044e12930
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>