python3: Use PYTHON3_PKG_BUILD to control default Python package build

python3: Use PYTHON3_PKG_BUILD to control default Python package build

This replaces the use of BUILD_VARIANT with PYTHON3_PKG_BUILD to opt
in/out of the default Python package build recipe (Py3Build/Compile).

PYTHON3_PKG_BUILD defaults to true (1), i.e. if a package includes
python3-package.mk, then by default it will set the package's
Build/Compile to Py3Build/Compile.

If PYTHON3_PKG_BUILD is set to 0 before python3-package.mk is included,
then Build/Compile will not be modified.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>