fastboot: Drop support for downloading to buffer ...

The option to download to a buffer instead of a file was introduced
because in some workloads it is required to have a contiguous image
in memory. With recent changes now ramfs can provide such a buffer
via memmap API even when it downloaded the data to a file. This makes
the explicit download to buffer option unnecessary, so remove it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fastboot: Use unique tempfile name ...

With fastboot over ethernet we now can have two instances of fastboot
running in parallel. Make sure that both instances use a different
temporary file to store data so that they do not influence each other.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fastboot: split generic code from USB gadget ...

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>