Add glibc-dev and linux-libc-headers-dev to gnomeos-contents-devel
Add libpng, iso-codes, lcms, json-c, gperf to runtime/devel
Resolve alternatives at do_rootfs time

Resolve alternatives of the form "toolname.package => toolname", such
as e.g. cat.coreutils -> cat (but not ones of the form busybox ->
cat).  This makes for cleaner images, and we throw away the
alternatives data anyway, so the links are useless.
Add gnomeos sdk and platform images

This will be used as the base for the gnome-os application work.
Its a lot more minimal and controlled than the other bases, as
we need to be very careful about what ends up in the platform.

In particular, there need not be any system management tools in there,
as this will not be used to boot systems.