Minimal config can be now build with:

./make-minimal BOARD=...

This is required because of Makefile.exports magic, which in its turn depends
on PROJ_CONF to be set correctly at the beginning of Makefile parsing at all
times. Instead of adding more and more workarounds for that, it's better to
just move minimal support to a separate wrapper.

Also, remove Zephyr 1.5 era cruft from Makefile, and add support for Zephyr's
"run" target which supercedes older "qemu" target in upstream.