Add old fastboot_protocol.txt

fastboot_protocol.txt used to live in bootable/bootloader/legacy
which was old, obsolete code.  However, fastboot_protocol.txt is
a useful file and belongs with the fastboot code.

Change-Id: Iea50c3219f9ff9dd0c394fc6028c456fa948ffd9
Signed-off-by: Scott Anderson <saa@android.com>