BaseTools: Move BuildOption parser out of build.py

BaseTools: Move BuildOption parser out of build.py

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

Build tool supports user to specify the conf folder.
To make the build options be evaluated at the beginning
of launching build, extract the buildoption function
from build.py to a new .py file.

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