Add MiniOS as a target

Add support for MiniOS as an additional target to Click. Like we discussed two weeks ago, this pull request already takes into account the changes you suggested i.e. usage of system toolchain, a bit of restyling,  docs. I don't exactly know how to deal with the dependencies (newlib, lwip, mini-os) but I don't include them to give more freedom to the developers. I added this information to the INSTALL file with instructions, in a ClickOS section.

Please have a look and let me know what do you think.

Cheers!