build-iso: update README build instructions for specific kernel types

Added three additional packages to the build requirements list in the
README file, one required and two optional, along with updating the
instructions for building an iso.  Currently, just using ./configure
on it's own will result in a 586-vyos being generated all the time, no
matter what flavour is specified for a kernel build.

Bug #277 http://bugzilla.vyos.net/show_bug.cgi?id=277