package/pkg-generic: handle host-xz as an extract dependency

package/pkg-generic: handle host-xz as an extract dependency

This moves the host-xz dependency handling from
DEPENDENCY_HOST_PREREQ to an extract dependency.

To achieve that, check-host-xz.mk fills in the
BR2_XZ_HOST_DEPENDENCY with host-xz if building a host-xz is
needed. The name BR2_XZ_HOST_DEPENDENCY has been chosen because it
matches the name BR2_CMAKE_HOST_DEPENDENCY already used in
check-host-cmake.mk.

The BR2_XZ_HOST_DEPENDENCY is added to all packages, except:

 - host-xz, because we would otherwise depend on ourself.

 - host-tar, because xz itself is delivered as a tarball, so we need
   to have host-xz depend on host-tar, and not host-tar depend on
   host-xz

 - host-skeleton, because we need to have host-xz depend on
   host-skeleton, and not the opposite.

In addition, we modify xz.mk to explicitly build host-xz without
ccache. We generally took the approach of building host-ccache *after*
all the extractors have been built.

[Peter: fix s/host-tar/host-xz/ typo]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>