This adds a new (optional) variable, PYPI_SOURCE_NAME, to pypi.mk.

For some PyPi packages (e.g. aiohttp_cors, click, django-compressor),
the name of the package and the source tarball name are slightly
different (usually by capitalisation or hyphen/underscore change).

This new variable is to make this difference explicit. PYPI_NAME is
meant for the "official" package name, whereas PYPI_SOURCE_NAME is meant
for the source tarball name.

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

This adds pypi.mk, which can be included in Python packages that
download their sources from PyPI, to auto-fill various PKG_* variables
based on the value of PYPI_NAME.

This makefile should be included after $(TOPDIR)/rules.mk but before
$(INCLUDE_DIR)/package.mk (and $(INCLUDE_DIR)/host-build.mk).

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