Makefile: handle header dependencies

Generate header dependency lists using gcc & include them for objects
already built.

Signed-off-by: Paul Burton <paulburton89@gmail.com>
Makefile: tidy up output

Silence the printing of commands by default, making that dependant upon
passing the V=1 argument for verbosity.

Signed-off-by: Paul Burton <paulburton89@gmail.com>
usb-test: abstract initialisation

Move the initialisation to a common directory in order to share it with
other tools.

Signed-off-by: Paul Burton <paulburton89@gmail.com>