usb_updater2: Add support for INJECT_ENTROPY command
usb_updater2: Add support for INJECT_ENTROPY command

Necessary for pairing support. Cleaner implementation to be moved
to hammerd.

BRANCH=none
BUG=b:38487027
TEST=Flash hammer. On host, reboot hammer to RO:
     usb_updater2 -r; sleep 0.5; usb_updater2 -s
     usb_updater2 -e (adds entropy)
     EC console: check that rollbackinfo shows secret is updated

Change-Id: I7b354c7bb1b6d4260939dc33fe6cb4ab4ecd50e0
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/513809
Reviewed-by: Chun-ta Lin <1fbd8af9e822d5561b2d88b84c2ffc9bb37fe394@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec