CLASS: PARTITION: do not force size of rootfs

With core-image-minimal, some issue occurs with forced size
of rootfs.

Signed-off-by: Christophe Priouzeau <christophe.priouzeau@st.com>

classes/st-partitions-image.bbclass

@@ -63,6 +63,8 @@ python __anonymous () {
                             bb.debug(1, "Set IMAGE_PARTITION_MOUNTPOINT to %s for %s partition image." % (items[2], items[0]))
                             d.setVar('IMAGE_PARTITION_MOUNTPOINT_pn-%s' % d.expand(items[0]), items[2])
                         if items[3]:
+                            if items[2]:
+                                # Mount point available, so we're dealing with partition image
                                 bb.debug(1, "Set IMAGE_ROOTFS_SIZE to %s for %s partition image." % (items[3], items[0]))
                                 d.setVar('IMAGE_ROOTFS_SIZE_pn-%s' % d.expand(items[0]), items[3])
                         else: