recovery: Fix the argument parsing from COMMAND_FILE.

Commit f0ed159f didn't ensure mounting
COMMAND_FILE (i.e. /cache) prior to reading. Also the first argument in
COMMAND_FILE should not be dropped (unlike the first "recovery" argument
in BCB message).

Bug: 33431888
Test: Push /cache/recovery/command and recovery imag parses the
      arguments successfully.

Change-Id: Ica3157a0bb03a2355315123354617eaf3fff15a4