Separate the all-monotouch-monodroid target into two all-monotouch and all-monodroid targets.

This makes it possible for the monotouch and the monodroid builds
to not depend on having the other installed.

It also makes the builds faster (by building less).
Merge pull request #361 from rolfbjarne/split-mobile-make-targets

Split the all-monotouch-monodroid target