vold: Format filesystems as exfat, if already exfat.

Change-Id: I1474fa3d3264e6466c2f74e91f16bd2515f50d06
Revenge of MiniVold (the sequel)

 * We'd like to utilize Vold's capabilities for performing various
   operations while in recovery mode.
 * Create a small, static library which can be linked into a single-call
   binary such as recovery.
 * Configurable helper paths.
 * Fixed various build warnings.

Change-Id: Ia4b929c2d7815d73f488ad406652f14b2a6aa4ce