duma: not available on non-MMU platforms

By design, building duma on Blackfin and other non-MMU platforms is
quite useless: it uses page mappings to detect buffer overrun and
use-after-free programming errors. Therefore, this commit simply marks
duma as not available on non-MMU platforms.

Fixes:

  http://autobuild.buildroot.org/results/951/9519d7abb45c6a789874ec6bb84418c31a98916b/



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