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/build/build.py

@@ -67,10 +67,6 @@ from AutoGen.AutoGen import CalculatePriorityValue
 ## standard targets of build command
 gSupportedTarget = ['all', 'genc', 'genmake', 'modules', 'libraries', 'fds', 'clean', 'cleanall', 'cleanlib', 'run']
 
-## build configuration file
-gBuildConfiguration = "target.txt"
-gToolsDefinition = "tools_def.txt"
-
 TemporaryTablePattern = re.compile(r'^_\d+_\d+_[a-fA-F0-9]+$')
 TmpTableDict = {}