boost: disable on NIOS 2 with broken toolchains

The current NIOS 2 toolchains are not capable of building Boost, so
let's disable it and its reverse dependencies. Even though it's not
strictly an architecture dependency, we use the <pkg>_ARCH_SUPPORTS
paradigm for this dependency, since it simplifies a lot handling all
boost reverse dependencies, and is anyway quite similar to an
architecture dependency since we don't display a comment about this
dependency.

Fixes:

  http://autobuild.buildroot.net/results/e119b1ef55c546e0d0598b85c46ceefa5c43d5a6/



[Peter: also update mpd comment]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>