Change the order of processing the command-line options and dynlinking

Previously, `mirage configure --xen` would called `Mirage.set_mode`
after processing `config.ml`.  This results in `Mirage.get_mode`
always returning `Unix` when queried from the configuration file,
which isn't helpful when trying to select which libraries to build.
Merge pull request #212 from avsm/fix-configure-load-order

Change the order of processing the command-line options and dynlinking