put demo objs into a separate variable
Expand OBJS and LDLIBS separately for app-tools substitution

No functional change
Separate LDFLAGS out of LDLIBS

XXX: Makefile needs to be put through a lawnmower
Make it easy to not link the networking stack.

Alternatively, could not link this file into the image, but it's easier
this way around.  Notably, the hypercalls should somehow be bundled
with the virtif driver, but that's build system rototill for another,
rainier day.
Speed up app-configure by a factor of many.

This is done by not linking in the full rump kernel at the configure
stage.  A cross-configure configure script will only do compile tests and
it does not really care about what is present in the kernel.  Since all
of the "libc" bits are in librump (apart from ones we handle directly on
top of minios), just linking in librump at the configure stage is enough.