Update build system

As currently set up, it takes far too much external knowledge to
generate builds for Fremantle (a set of EXTRA_CPPFLAGS as well as
EXTRA_LDFLAGS); this is because the Makefiles were originally written
when there was only one target system (Diablo).

To fix this, provide new "diablo" and "fremantle" targets that can be
used to build the appropriate browser-switchboard without further
configuration, and make the "all" target print out a help message.  For
the config UI, provide new "fremantle-hildon-app" and "fremantle-plugin"
targets, and rename the existing targets to "diablo-hildon-app" and
"diablo-plugin" to match.