build: add rules for nesting

build: add rules for nesting

This adds the 'magic' rules that take care of subdirectories.
The subdirectory makefiles in the source tree are not complete; they
only define some variables (listed in nested-vars) according to the
configuration.

The magic rules descend into subdirectory makefiles and gather the
evaluated values of those variables.  The values from all subdirectories
are joined together, each prefixed with the subdirectory name, and used
by the "real" makefiles.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>