bootloader_message: allow devices to override

* Some devices do not have a misc partition or something that can be
  used as a surrogate, but we still need a place to write the recovery
  commands to. In these cases, allow a device tree to override the
  default aosp behavior and instead do such things as use a file on
  /cache to handle this.

* NOTE: Your device must also specify BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE
  if you plan to use /cache/recovery/command as this is used to tell
  android to use this dedicated /cache partition rather than symlinking
  it to /data/cache

Change-Id: I254c28a1e4d9b8df9c0e5d9ebcc087b58a45d5a3