xen/test/livepatch: Regularise Makefiles

xen/test/livepatch: Regularise Makefiles

In xen/test/livepatch/Makefile:

  Provide a `build' target, as most of the
  subdir-invoking Makefiles elsewhere expect.

In xen/test/Makefile:

  Replace the two open-coded targets with a generalised pattern rule
  which descends into each of SUBDIRS.  This allows `install' to work
  too (it is already supported by xen/test/livepatch/Makefile).

  Provide an explicit default target of `tests', and an `all' target
  (which is conventional).

  Suppress entry into the xen/test/livepatch subdir when we are
  building for i386, since the 32-bit hypervisor is not supported any
  more and we can't build livepatches for it either.

After this, the xen/test subdirectory is somewhere were make can be
invoked in the way which is conventional for xen.git/xen/ subdirs.

None of this is yet invoked from the top-level Makefile.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>