rc: start drivers for attached BeagleBone Capes

BeagleBone Capes add additional hardware to the BeagleBone to
improve functionality.

This patch probes the third i2c bus for capes and loads the
appropriate drivers for each detected cape. Currently only
the 'BeagleBone Weather' cape is supported.

Change-Id: Id8c133810db6de7c21625c2d5a794b8874673a0f