Add u-boot flag to flash script

- Add '-u' flag to the flash script, to use u-boot for the flash instead
of LK.
- For the moment, this is primarily to help test infra, so that the
board doesn't switch from our crypto-chip derived serial to the
hard-coded LK serial mid-flash script.

Change-Id: I5cf6563567713ade634673172f98dedbf2f5d950
1 file changed