libfdt: overlay_merge: remove resolved symbols

Merging two overlay blobs will involve merging contents of nodes such as
__symbols__. Not all properties need to be merged however at the end of merge
process. overlay_symbol_update() may already have updated base blob's
__symbols__ node to reflect new position of some nodes from overlay blob. Remove
such symbols from overlay blob's __symbols__ node, to prevent a subsequent merge
of __symbols__ node of both blobs from creating duplicate entries representing
same node.

Change-Id: I8ce45011db3f565d903ace922870a2c2616a560c
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>