cmd: fastboot: Rework fastboot dependency

Fastboot need a bunch of options to be operating properly, such as the
g_dnl gadget, the fastboot command, and some options that make sense. Since
fastboot is now part of Kconfig, make sure we have them right.

That will also reduce the boilerplate in the defconfigs.

Reviewed-by: Łukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>