If xzcat is not present on the host system, buildroot bails out early asking
the developer to install it (xzcat is now a DL_TOOLS_DEPENDENCY)
Conversely, when BR2_TARGET_ROOTFS_CPIO_XZ is enabled, then host-xz is a
build dependency, and no manual action is required from the developer.
Because the second approach is nicer, also build host-xz when xzcat is not
available, using the host-prerequisite and suitable-host-pkg mechanisms,
already used for tar.
Signed-off-by: Thomas De Schampheleire <firstname.lastname@example.org>
Signed-off-by: Peter Korsgaard <email@example.com>