libmnflash: use autoconf

Just autoconf, no automake or - deity beware - libtool.

The idea of using this is to let the user define

* installation paths
* if a shared library should be built and used.

and also check for libelf and libusb.

Also added a primitive "autogen.sh" to generate "configure", which must be run
on a git checkout.
libmnflash: add the configure script to git.

With this added, "git --archive" can create a usable distribution archive,
otherwise we had to rely on the user having the right autoconf version
installed.