Fix multiple container installation failing

When new container image add into lxc-host-image-demo, existing
image will not build.

In case of:
CONTAINER_IMAGES ?= "agl-container-cluster:guest-image-cluster-demo \
                     agl-container-ivi:guest-image-ivi-demo \
"

agl-container-ivi:guest-image-ivi-demo will be building,
but agl-container-cluster:guest-image-cluster-demo will not be building.

This patch fix issue for the build dependency creation to guests.

Bug-AGL: SPEC-4109

Signed-off-by: Naoto Yamaguchi <naoto.yamaguchi@aisin.co.jp>
Change-Id: I2ed6f91f0d4db90f337a6148d6630f85c1b5c97d

meta-agl-lxc/recipes-platform/images/lxc-host-image-minimal.bb

@@ -22,7 +22,7 @@ python __anonymous() {
     for c in (d.getVar('CONTAINER_IMAGES') or "").split():
         (mc, image) = c.split(':')
         dependency = 'mc::' + mc + ':' + image + ':do_image_complete'
-        d.setVarFlag('do_rootfs', 'mcdepends', dependency)
+        d.appendVarFlag('do_rootfs', 'mcdepends', ' ' + dependency)
 
         # Assume there is a X-lxc-config package for guest-image-X
         config = image