bmpblk: Update README file.

The build procedure has been slightly changed (although the old workflow still
works fine):

 - "Adding new board" now needs only boards.yaml. No more scripting hacks.
 - flashrom syntax (-p <target>) is changed.

BUG=none
TEST=none, simply revising text file.

Change-Id: Iddd7c3d0fa0391cd6217f2424b065bfaed708371
Reviewed-on: https://chrome-internal-review.googlesource.com/148187
Reviewed-by: Hung-Te Lin <[email protected]>
Tested-by: Hung-Te Lin <[email protected]>
Commit-Queue: Hung-Te Lin <[email protected]>
1 file changed