fdtoverlay: Rework output allocation

At present the fdtoverlay tool allocates space for its output based on a
calculation of the worse case size based on the size of the input blobs.

Except.. that certain edge cases with very long target paths can actually
exceed that "worst case" calculation.

This reworks the code to instead dynamically reallocate the output buffer
if we run out of space at any point.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>