BaseTools/GenMake: Use ToolDefinition as fallback option

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3653

Use the value set in tools_def.txt when the makefile type is
not explicitly set via BuildOption. This allows to have a
valid default makefile name instead of an empty string.

Also use GMAKE_FILETYPE instead of hard-coded "gmake".

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Chris Jones <christopher.jones@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Source/Python/AutoGen/GenMake.py

@@ -2,7 +2,7 @@
 # Create makefile for MS nmake and GNU make
 #
 # Copyright (c) 2007 - 2021, Intel Corporation. All rights reserved.<BR>
-# Copyright (c) 2020, ARM Limited. All rights reserved.<BR>
+# Copyright (c) 2020 - 2021, Arm Limited. All rights reserved.<BR>
 # SPDX-License-Identifier: BSD-2-Clause-Patent
 #
 
@@ -177,11 +177,11 @@ class BuildFile(object):
 
         MakePath = AutoGenObject.BuildOption.get('MAKE', {}).get('PATH')
         if not MakePath:
-            self._FileType = ""
-        elif "nmake" in MakePath:
+            MakePath = AutoGenObject.ToolDefinition.get('MAKE', {}).get('PATH')
+        if "nmake" in MakePath:
             self._FileType = NMAKE_FILETYPE
         else:
-            self._FileType = "gmake"
+            self._FileType = GMAKE_FILETYPE
 
         if sys.platform == "win32":
             self._Platform = WIN32_PLATFORM