rootfs_rpm.bbclass: save rpmlib rather than remove it

The rpmlib was removed when images that add
"remove_packaging_data_files" to ROOTFS_POSTPROCESS_COMMAND, which would
make the increment rpm image generation doesn't work in the second
build, since list_installed_packages would get incorrect value in the
second build, move the rpmlib to ${T} rather than remove it, and move it
back when INC_RPM_IMAGE_GEN =1.

[YOCTO #2440]

(From OE-Core rev: c30e79510c06701f10f659eedaa0fe785538ac17)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Elizabeth Flanagan <elizabeth.flanagan@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/rootfs_rpm.bbclass

@@ -154,7 +154,11 @@ EOF
 }
 
 remove_packaging_data_files() {
-	rm -rf ${IMAGE_ROOTFS}${rpmlibdir}
+	# Save the rpmlib for increment rpm image generation
+	t="${T}/saved_rpmlib/var/lib"
+	rm -fr $t
+	mkdir -p $t
+	mv ${IMAGE_ROOTFS}${rpmlibdir} $t
 	rm -rf ${IMAGE_ROOTFS}${opkglibdir}
 }