ShellPkg: Add check for "dump" parameter in "bcfg" command

ShellPkg: Add check for "dump" parameter in "bcfg" command

When user uses the command "bcfg driver|boot [dump [-v]]",
the number of command line value parameters (doesn't include the
flag) must be three. We can add this point to check whether using
this command correctly.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hpe.com>