libfdt: overlay_merge: Rename fragments

When merging two overlay blobs, fragment nodes whose target node can't be found
in base blob would need to be retained as-is (including the fragment names) in
the combined blob. Such unresolved symbols will also need to be listed in
__fixups__ section of combined blob. This could lead to name comflicts in
combined blob (two nodes with same name/path such as /fragment@0).

To avoid such name conflicts in combined blob, rename all fragment@xyz in
overlay blob as fragment@xyz+delta, where delta is the maximum count of fragment
nodes found in base blob

Change-Id: I987f60ceed1c5e05279b0c4ff998affd069d70fd
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>