install apk keys and binfmt data with setup.py

So far we've only installed the Python source code in the pmb folder. But pmbootstrap needs the apk package signing keys as well as the binfmt data file to work correctly, so this commit moves all of them to pmb/data and changes setup.py and MANIFEST.in to install them as well. Also the code and test case have been adjusted to use the new paths.

How to test

WARNING: this will install pmbootstrap system wide (creates /usr/local/bin/pmbootstrap and /usr/local/lib/python*/site-packages/pmbootstrap*)

$ sudo setup.py install
$ pmbootstrap -y zap
$ pmbootstrap build --arch=armhf --force hello-world

(The last command makes use of the binfmt data as well as the alpine keys to set up the chroots.)

Fixes #1661 (closed).