Allow specifying max text colors to reduce output size.

Some boards may have too many resources that we have to decrease text images
quality to fit into GBB.

BUG=chrome-os-partner:25024
TEST=(before) make zako # output size = 1078595
     (after) make zako # output size = 940615

Change-Id: I3740c41ad28c5dd42cba6b9604974dc0f9f8f1d3
Reviewed-on: https://chrome-internal-review.googlesource.com/170574
Tested-by: Hung-Te Lin <hungte@google.com>
Reviewed-by: Bill Richardson <wfrichar@google.com>
Commit-Queue: Hung-Te Lin <hungte@google.com>