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>

support/dependencies/check-host-xzcat.mk

@@ -0,0 +1,7 @@
+# XZCAT is taken from BR2_XZCAT (defaults to 'xzcat') (see Makefile)
+# If it is not present, build our own host-xzcat
+
+ifeq (,$(call suitable-host-package,xzcat,$(XZCAT)))
+  DEPENDENCIES_HOST_PREREQ += host-xz
+  XZCAT = $(HOST_DIR)/usr/bin/xzcat
+endif

support/dependencies/check-host-xzcat.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+candidate="$1"
+
+xzcat=`which $candidate 2>/dev/null`
+if [ ! -x "$xzcat" ]; then
+	xzcat=`which xzcat 2>/dev/null`
+	if [ ! -x "$xzcat" ]; then
+		# echo nothing: no suitable xzcat found
+		exit 1
+	fi
+fi
+
+echo $xzcat