MMU: Simplify tee_mmu_switch function

Replaced direct CP15 instructions with macros instead and removed some
unnecessary code.
Make license terms more explicit

It fixes #37 "README.md is unclear about license"
Makefile variables $(*-dir) should not have a trailing slash

As a general rule, Makefile variables that are directories should not have
a trailing slash, and should be used as: $(some-dir)/some-file rather than
$(some-dir)some-file. This is more readable.