xzcat: treat as host prerequisite and build if needed

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 <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>