recovery: Fix the argument parsing.

Commit f0ed159f unintentionally changed
the behavior when parsing the arguments from BCB and command file. It
should only parse the command file for arguments if BCB doesn't supply
any.

As a result, it shows the following from the log:
I:Got 2 arguments from boot message
I:Got 3 arguments from /cache/recovery/command

Test: Set arguments in BCB and command file both. It should only use the
      arguments from BCB.

Change-Id: Idf371137aa9177f1b2dfdfecf0c9f46959d2ee76