edksetup.sh: Fix the Issue of PYTHON_COMMAND Un-Configurable

- With the commit e6447d2a08f5ca585816d093e79a01dad3781f98 introduced,
  there do not have chance for caller to configure the PYTHON_COMMAND
  environment variable outside of this script.

- All the configured value would be assigned into "python3" forcedly,
  without checking the environment variable is set or not.

- This patch included the below changes,
  - Check the "PYTHON_COMMAND" is set or not before assigning
    the default value "python3" on it.
  - Rename the function naming into "SetupPythonCommand" to align
    its functionality.

Signed-off-by: Jason1 Lin <jason1.lin@intel.com>

edksetup.sh

@@ -103,14 +103,22 @@ SetupEnv()
   fi
 }
 
-SetupPython3()
+SetupPythonCommand()
 {
+  #
+  # If PYTHON_COMMAND is already set, then we can return right now
+  #
+  if [ -n "$PYTHON_COMMAND" ]
+  then
+    return 0
+  fi
+
   export PYTHON_COMMAND=python3
 }
 
 SourceEnv()
 {
-  SetupPython3
+  SetupPythonCommand
   SetWorkspace
   SetupEnv
 }