BaseTools: Remove hard-coded strings for target and tools_def

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>