python-dns: add $(eval $(call PyPackage,python-dns))

Should be included by Python packages if they want to run
Python-package-specific build rules.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python-dns: update compile rule and add PyPackage rules

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python-dns: update PKG_NAME

PKG_NAME should be the name of the actual OpenWRT package, and not
the name of the original package name.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>