It turns out that recent subversion clients are more nitpicking with respect to
the usage of 'svn update'. Formerly it was ok to just checkout a toplevel
directory of some repository, and then update that parts in the repo that are
needed. This is used in the preparation makefile in 'base-foc' to just checkout
a small part of the L4Re software stack. Recent clients complain about using
update with some remote target, so we've to use 'checkout' here too.
This commit fixes#84.
