The fastboot specification describes other protocols beyond USB. Allow
these to reuse the generic parts of the existing fastboot code when they
are implemented.

Most of the changes in common/fastboot.c are due to the renaming of struct
f_fastboot *f_fb to struct fastboot *fb.

Signed-off-by: Edmund Henniges <eh@emlix.com>
Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>