bvb: Update path to key since repo was moved.

We recently moved Brillo verified boot from system/bvb to external/bvb
so update the path accordingly.

TEST=Build with 'BOARD_BVB_ENABLE := true' succeeds.
BUG=29099910

Change-Id: I66f3b90e23d6b2afc09f81af571ede7b573325e9
1 file changed