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

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

BRANCH=none
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
        DEVELOPER_BOOT_EXTERNAL
     3. Plug in invalid usb, then screen changes to
         DEVELOPER_INVALID_DISK
     4. Unplug usb, then screen changes back to
        DEVELOPER_BOOT_EXTERNAL
     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