meta-phosphor: Make image-overlay an optional feature

Previously, obmc-bsp-common.inc added "overlay" to the IMAGE_FSTYPES
but this only has meaning if the 'image-overlay' class is used.  The
obmc-phosphor-image class explicitly includes 'image-overlay' when
appropriate but no other images (ie. ones that come with yocto) do.

Changed obmc-bsp-common to instead enable an obmc-phosphor-image feature
by setting OBMC_PHOSPHOR_IMAGE_OVERLAY=1. Then in obmc-phosphor-image we
add "overlay" to the IMAGE_FSTYPES only when the feature is enabled.

This will fix both non-phosphor images on hardware platforms and all images
on simulation platforms.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meta-phosphor/classes/obmc-phosphor-image.bbclass b/meta-phosphor/classes/obmc-phosphor-image.bbclass
index d3960c7..2d97cde 100644
--- a/meta-phosphor/classes/obmc-phosphor-image.bbclass
+++ b/meta-phosphor/classes/obmc-phosphor-image.bbclass
@@ -48,9 +48,10 @@
 
 OBMC_IMAGE_EXTRA_INSTALL ?= ""
 
-def build_overlay(d):
-        if bb.utils.contains("IMAGE_FSTYPES", "overlay", "overlay", "0", d) != "0":
-                return "image-overlay"
-        return ""
+def image_overlay_enabled(d, ifEnabledStr):
+        if d.getVar('OBMC_PHOSPHOR_IMAGE_OVERLAY', True) != "1":
+            return ""
+        return ifEnabledStr
 
-inherit ${@build_overlay(d)}
+IMAGE_FSTYPES += "${@image_overlay_enabled(d, "overlay")}"
+inherit ${@image_overlay_enabled(d, "image-overlay")}