libglib2: needs threads

This commit adds a dependency of the libglib2 package on thread
support in the toolchain, since upstream libglib2 doesn't build
without thread support. The commit is rather large as it involves
propagating the dependency on thread support to all reverse
dependencies of the libglib2 package.

[Thomas: squash all patches into one, make a few minor fixes, the most
important one being to not add comments about MMU requirement when a
package doesn't work on !MMU platforms.]

Signed-off-by: Spenser Gilliland <spenser@gillilanding.com>