eap_peer: create a libeap library, with header files and pkg-config [v2]

eap_peer: create a libeap library, with header files and pkg-config [v2]

This adds infrastructe in src/eap_peer to make libeap.so and install
the needed header files and pkg-config files.

Now, this is quite dirty and probably not what we want in the long
term, but serves as an starting point:

 - we don't build from the wpa_supplicant directory because the
   objects the .so have to be built with -fPIC. So if you need to
   build both the binary and the library:

   make -C wpa_supplicant
   make -C src/eap_peer clean
   make -C src/eap_peer

   As I said, it's dirty -- we'd need either wpa_supplicant linking
   against the library properly (but that seems not to be desirable)
   or a multiple object build approach ala automake.

 - need to use 'override CFLAGS' in src/eap_peer/Makefile, otherwise
   any CFLAGS setting will kill the build infrastructure. I miss
   AM_CFLAGS.

 - adds 'eap_register_methods()' that will register every compiled in
   method.