usb/gadget: add the fastboot gadget

This patch contains an implementation of the fastboot protocol on the
device side and documentation. This is based on USB download gadget
infrastructure. The fastboot function implements the getvar, reboot,
download and reboot commands. What is missing is the flash handling i.e.
writting the image to media.

v3 (Rob Herring):
This is based on http://patchwork.ozlabs.org/patch/126798/

 with the
following changes:
- Rebase to current mainline and updates for current gadget API
- Use SPDX identifiers for licenses
- Traced the history and added missing copyright to cmd_fastboot.c
- Use load_addr/load_size for transfer buffer
- Allow vendor strings to be optional
- Set vendor/product ID from config defines
- Allow Ctrl-C to exit fastboot mode
v4:
- Major re-write to use the USB download gadget. Consolidated function
code to a single file.
- Moved globals into single struct.
- Use puts and putc as appropriate.
- Added CONFIG_USB_FASTBOOT_BUF_ADDR and CONFIG_USB_FASTBOOT_BUF_SIZE to
set the fastboot transfer buffer.
v5:
- Add CONFIG option documentation to README
- Rebase using new downloader registration

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>