ShellPkg: Fix 'EfiShellExecute' doesn't get command status correctly.

1. Add a new  function 'RunShellCommand' to return command status, thus 'EfiShellExecute' can get the command return status of 'CommandLine'.
2. Refine the code logic of 'EfiShellExecute' to make the new image of shell be loaded only if  'Environment' isn't NULL.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Signed-off-by: Jin Eric <eric.jin@intel.com>
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18664 6f19259b-4bc3-4df7-8a09-765794883524

ShellPkg/Application/Shell/Shell.h

@@ -293,6 +293,25 @@ RunCommand(
   IN CONST CHAR16   *CmdLine
   );
 
+/**
+  Function will process and run a command line.
+
+  This will determine if the command line represents an internal shell 
+  command or dispatch an external application.
+
+  @param[in] CmdLine      The command line to parse.
+  @param[out] CommandStatus   The status from the command line.
+
+  @retval EFI_SUCCESS     The command was completed.
+  @retval EFI_ABORTED     The command's operation was aborted.
+**/
+EFI_STATUS
+EFIAPI
+RunShellCommand(
+  IN CONST CHAR16   *CmdLine,
+  OUT EFI_STATUS    *CommandStatus
+  );
+
 /**
   Function determines if the CommandName COULD be a valid command.  It does not determine whether
   this is a valid command.  It only checks for invalid characters.