bot_mk1 "braking bad" fix

Turned out the issue was due to an overly aggressive max brake setting in the bot_mk1 setup. Reduced the bot_mk1's max braking value from 1e6 to 2e5 (twice the drive value) and the problem went away. I'm good with this fix for alpha, but we may need to revisit the math in diff_steer later to figure out where we were losing it and if it matters.

Unit tested and ready to go, this completes coding and unit test for  bug 11.