Make the libapk and lua module optional at buildtime and off by default.
This is because enabling libapk requires that mkinitfs also copies the
libapk.so or the initramfs will not work.

To build libapk:

 make LIBAPK=yes

To build lua module (will enable LIBAPK=yes):

 make LUAAPK

To link apk to libapk (will enable LIBAPK=yes):

 make SHARED_LIBAPK=yes

All the above applies to make install target as well.