mongrel2: disable polarssl assembly as we do in the polarssl package

Fixes:
http://autobuild.buildroot.net/results/a92292a1defd441938d3de6c5049473d520ebc4e/
http://autobuild.buildroot.net/results/f3db555dc29e0e0e87762fcace913853ffe2b176/



mongrel2 bundles polarssl so it has the same problem as the polarssl
with the assembly optimization, so let's use the same solution here.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>