destdir support for Rumprun

[Antti Kantee]

Hi,

I'm tweaking the build system (the fun never ends) to allow installation 
of the Rumprun toolchain and libs into an arbitrary directory, the idea 
of course being that you can then tar up that directory for binary 
packages.  I'm currently targeting for it to be possible to install one 
configuration; having e.g. x86_64-hw and x86_64-xen in the same destdir 
is out of the immediate scope (even if in the eventual scope).

Naturally, some amount of rototilling is required.  As usual, 
"build-rr.sh" is the only official way to build things.  However, it's 
important for me to keep the developer shortcuts working too.  So, if 
over the course of the rototillage you notice some shortcuts breaking, 
ping the list, and we'll see what to do about it.

The user-visible change will be installing app-tools to destdir/bin.  If 
someone wants to write the compat lint to keep app-tools also where they 
currently are in addition to destdir, let me know in the next day or so. 
 Otherwise, we'll just have a flag day.

I'll send another email when the initial part of the rototill is 
complete (a day or few, depending on how boring fighting with the build 
system gets).