efl: disallow on non-MMU systems

The libeina library uses the madvise() system call, that isn't
available on non-MMU systems. Also, several other components of EFL
use fork(). Therefore, the easiest solution is to simply disallow the
EFL as a whole on non-MMU systems.

Fixes:

  http://autobuild.buildroot.org/results/ad9/ad90baa5e07569308a7e2b2510b67c5b2a563b44//



Thanks to Ryan Barnett for helping in the investigation!

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>