Retry reading mcv4 fw version in case of failure

On a cold boot, the motor rail is not initially on.  It is only
possible to communicate with the motor controllers when the motor rail
is powered-up.  There is a non-zero period of time between the motor
rail being turned on and the point when we can communicate with the
motor controller.

We were trying to read the firmware version out of the motor
controller too soon, before it had finished powering-up.  This patch
makes it retry the process of reading the firmware version up to 10
times before giving up.

This means that on cold-boot, the message "Warning: Motor board is not
running the expected firmware" will no longer appear unless the motor
board is actually running the wrong firmware.

Change-Id: I7c2eb050d97166cdb40f6fcff309d1ee0c4945e2
Dump detected USB devices to log on start-up

Since both the motor controllers and jointio replacement are now USB,
they don't appear in the SRIC device listing on start-up.  This patch
adds listing of the detected USB devices.  It also adds a "serialnum"
property to the Motor and Ruggeduino objects so that their serial
numbers can be displayed in this listing.

Change-Id: I23a310d644d5997d390652d0325e4e800bcb96ad
Remove motor and jointio firmwares

We no longer use these boards, so there's no point in keeping the
firmwares here.

Change-Id: I8b27c8b092614d9de3d26f2a89c75f1f3aad30a1
Stop fw splash from showing when there's no update

The firmware-update splash screen would always display for a short
period of time upon boot, regardless of whether an update was
occuring.  This patch uses the power board's SRIC vbuf to determine
whether an update is necessary, and then starts the splash screen if
appropriate.

Change-Id: I676f7da4e938d89b1dc55ba889901b3a94a43f99