Add a hunter for USB which enumerates the bus to find new bootdevs.
Update the tests and speed up bootdev_test_prio() while we are here, by
dropping the USB delays.
Signed-off-by: Simon Glass <sjg@chromium.org>
The code in these functions turns out to often be the same. Add a default
get_bootflow() function and allow the drivers to select it by setting
the method to NULL.
This saves a little code space.
Signed-off-by: Simon Glass <sjg@chromium.org>
Add a bootdev driver for USB host. It can use the distro boot mechanism to
locate a file, or any other available bootmeth.
Signed-off-by: Simon Glass <sjg@chromium.org>
