fastboot: use constants.h values

Cleanup to utilize all the manifest values in constants.h.

If the cli command _and_ the protocol name match, use a common
source of convenient truth.  This should set a pattern for future
additional commands.  When the command and the protocol differ,
we want to introduce resistance as it results in confusion and
maintenance issues.

Test: compile
Change-Id: Idad413c63cbbfcb6e851856105a5d5a9ef53ef29