Pull out recovery filesystem into a separate target.

Add a timestamp file for the recovery filesystem contents, and modify
recovery image targets to depend on the timestamp file and include only
instructions for building the actual image. Build-tested on encore and one
(one for each type of recovery.img supported), successfully builds
recovery and recovery.img.

Change-Id: I2cf193e8f8bf93edbd25c6057c5c3e6aa90e7f77