Fix building libprop as a host tool library on platforms that don't have
the Matt Thomas rbtree:

- Include rb.c in libnbcompat, and provide a nbcompat sys/rbtree.h
header.
- Make sure libprop's so

Fix building libprop as a host tool library on platforms that don't have
the Matt Thomas rbtree:

- Include rb.c in libnbcompat, and provide a nbcompat sys/rbtree.h
header.
- Make sure libprop's source file include prop_object_impl.h before
anything else, and pull in nbtool_config.h from there.

Tested by simulating such a host system by renaming the host's
<sys/rbtree.h> out of the way (which reproduced the build failure)
and verifying that the host-tool installboot contained the rb_*
functions in its own .text segment.

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Add support for libprop as a host tool library.