python-packages: Disable parallel build when host pip is needed
This adds PKG_BUILD_PARALLEL:=0 to packages that depend on host Python
packages (HOST_PYTHON3_PACKAGE_BUILD_DEPENDS), because installing
packages with multiple concurrent pip processes can lead to errors or
unexpected results[1].
This also:
* Move HOST_PYTHON3_PACKAGE_BUILD_DEPENDS definitions to before
python3-package.mk is included
* Update Python folder readme to include PKG_BUILD_PARALLEL:=0
[1]: https://github.com/pypa/pip/issues/2361
Signed-off-by: Jeffery To <jeffery.to@gmail.com>
[cffi is one of those packages that needs a host-side package installed].
This works reasonably well in the current OpenWrt build system, as binaries get built for this package and get installed in the staging-dir `$(STAGING_DIR)/usr/lib/pythonX.Y/site-packages`.
`PKG_BUILD_PARALLEL:=0` is necessary because installing packages with multiple concurrent pip processes can lead to [errors or unexpected results](https://github.com/pypa/pip/issues/2361).