vboot/ui: add screens for dev boot from external disk

Add polling screens for booting from external disk in dev mode.

BUG=b:146399181, b:158973903
TEST=make runtests
TEST=emerge-puff depthcharge
TEST=1. Enter developer mode screen
     2. Select "Boot from external disk", then screen changes to
     3. Plug in invalid usb, then screen changes to
     4. Unplug usb, then screen changes back to
     5. Plug in valid usb, then device boots successfully

Cq-Depend: chromium:2245066
Change-Id: I633f16ca0d92eaf27eb3b7630ee61b2044942741
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2246298
Reviewed-by: Joel Kitching <kitching@chromium.org>
2 files changed