build(fiptool): add object dependency generation

build(fiptool): add object dependency generation

The object target in the fiptool Makefile only depends on the
corresponding source file so it won't rebuild the object, if a header
file used by the source file is changed.

To make it rebuild the object file for both source and header file
changes, a dependency file will now be generated for each object and
included in the Makefile.

Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>
Change-Id: I0468c6e9c54126242150667268d471f28e011b0d