Avoid to re-generate string table from ufdt to fdt

String table contains the strings of all property name in a fdt.

The ufdt_apply_overlay() converts two dtbs from fdt to ufdt,
overlays, and converts merged ufdt to fdt. These operations
shouldn't create new peroperty names, so we can just re-use the
string tables in original dtbs, and just copy them into merged
fdt. This solution can enhance a lot of performance for device
tree overlaying.

To avoid the error that some users could use string offset in
string table, the solution also give a same string offset for
the name properties by ufdt_prop_dict;

Futher, the patch also removed unused header files after
changing algorithm.

Bug: 35255584
Test: ./tests/run_tests.sh
Change-Id: Id422730115531bd20d21117285291bdd860915ff
(cherry picked from commit 1be68ae5)