EmbeddedPkg/FdtLib: incorporate missing overlay support

Commit a099239015eb ("EmbeddedPkg/FdtLib: Update FdtLib to v1.4.5")
updated our FdtLib implementation to a more recent upstream version,
but omitted fdt_overlay.c due to its ambiguous licensing situation.
This has been resolved now, so take the latest upstream version of
the file (which received no other modifications in the mean time)
and add it to FdtLib.

Note that fdt_overlay.c has a dependency on strtoul() which EDK does
not provide natively, so add a private implementation that wraps the
BaseLib routines that do roughly the same.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>