fix image generation when signing is enabled

When using signing openssl-native is required. Since pyro the sysroot
is deployed per recipe, so do_swuimage also requires
do_prepare_recipe_sysroot.

Signed-off-by: Christian Andersen <c.andersen@kostal.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>

classes/swupdate.bbclass

@@ -199,5 +199,5 @@ python do_swuimage () {
 COMPRESSIONTYPES = ""
 PACKAGE_ARCH = "${MACHINE_ARCH}"
 
-addtask do_swuimage after do_unpack before do_install
+addtask do_swuimage after do_unpack after do_prepare_recipe_sysroot before do_install
 addtask do_createlink after do_swuimage before do_install