Make tool to check power board FW revision (ros ticket #4184)

[ahendrix]

We should be able to add an option to the power node2 and get the FW rev printed to stdout. Using pr2-systemcheck, we can verify that we have the right FW.

trac data:

• Owner: cmeyers
• Reporter: watts
• Reported at: Thu Apr 15 21:00:36 2010
• URL: https://code.ros.org/trac/wg-ros-pkg/ticket/4184

[ahendrix]

[watts] This looks like it will be very, very hard to do with existing code.

We should do this eventually, but the power_node will have to be rewritten.