ramsmp: not available on !MMU, needs fork

Fixes:

  http://autobuild.buildroot.org/results/aaa/aaa7f8e5ec72ce69805d6c1a5cde5ff47e1c118e/



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