ShellPkg: Add Shell invocation option '-exit'

According to Shell spec 2.2 '-exit' invocation option is used to specify
that after running the command line when launched, the UEFI Shell must
immediately exit.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>

ShellPkg/Application/Shell/Shell.c

@@ -1022,7 +1022,7 @@ ProcessCommandLine(
       }
     } else if (UnicodeCollation->StriColl (
                                    UnicodeCollation,
-                                   L"-_exit",
+                                   L"-exit",
                                    CurrentArg
                                    ) == 0) {
       ShellInfoObject.ShellInitSettings.BitUnion.Bits.Exit         = TRUE;