remove 4MB constraint from mikrotik profile

Fixes #186.

Buildbot: http://buildbot.berlin.freifunk.net/builders/ar71xx_mikrotik/builds/361
Buildbot images should end up here: http://buildbot.berlin.freifunk.net/buildbot/unstable/ar71xx_mikrotik/361

@faust2k: can you test once the image is available? I don't have a mikrotik board... :smile: