MokoMakefile disabling of "svnnow" and new force-rebuild target

[Rod Whitby]

I have modified MokoMakefile to disable the "svnnow" functionality
(which rebuilds all openmoko-specific packages every time you build),
and have added a 'force-rebuild' target to the Makefile which lets you
do the same thing, but at a time of your choosing.

Whether or not you agree with the OpenMoko core dev's decision to use
the "svnnow" functionality, this now gives you the opportunity to have
stable builds (which only rebuild daily from SVN rather than every time
you start a build), but also force a rebuild of everything that would
have otherwise been marked as 'svnnow' (the force-rebuild target reads
the preferred-versions file to work out the specific set of packages to
clean).

There is a whole other debate about the "svnnow" functionality itself,
and whether or not it is appropriate for external devs, or whether or
not it is the "correct" way to use OpenEmbedded.  I'm not going to get
into that debate, as I can see both sides of the debate as having valid
viewpoints for different situational requirements.

-- Rod