BaseTools: Remove hard-coded strings for target and tools_def

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

The "target.txt" and "tools_def.txt" filenames are hard-coded
at some places when global definitions are available at:
BaseTools/Source/Python/Common/TargetTxtClassObject.py:
DefaultTargetTxtFile
and
BaseTools/Source/Python/Common/ToolDefClassObject.py:
DefaultToolsDefFile

Use these global definitions instead.

Also remove the unused gBuildConfiguration and gToolsDefinition
variables from build.py

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/TargetTool/TargetTool.py

@@ -17,6 +17,7 @@ import Common.BuildToolError as BuildToolError
 from Common.DataType import *
 from Common.BuildVersion import gBUILD_VERSION
 from Common.LongFilePathSupport import OpenLongFilePath as open
+from Common.TargetTxtClassObject import gDefaultTargetTxtFile
 
 # To Do 1.set clean, 2. add item, if the line is disabled.
 
@@ -25,7 +26,7 @@ class TargetTool():
         self.WorkSpace = os.path.normpath(os.getenv('WORKSPACE'))
         self.Opt       = opt
         self.Arg       = args[0]
-        self.FileName  = os.path.normpath(os.path.join(self.WorkSpace, 'Conf', 'target.txt'))
+        self.FileName  = os.path.normpath(os.path.join(self.WorkSpace, 'Conf', gDefaultTargetTxtFile))
         if os.path.isfile(self.FileName) == False:
             print("%s does not exist." % self.FileName)
             sys.exit(1)