BaseTools: Singleton the object to handle build conf file

BaseTools: Singleton the object to handle build conf file

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1875

The build config files are target.txt, build rule, tooldef
During a build, the config is not changed, so the object to
handle them need to be singleton.

Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Liming Gao <liming.gao@intel.com>