overlay: remove OBMC_PHOSPHOR_IMAGE_OVERLAY

Solve the problem resulting in the addition of this variable
differently, such that it can be removed and image type
selection can be selected in the standard way using IMAGE_FSTYPES.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Change-Id: Ice94e8f278f00b8b4d344244bfb87dddeb173a23
diff --git a/conf/distro/openbmc-phosphor.conf b/conf/distro/openbmc-phosphor.conf
index 9f8bad2..924e119 100644
--- a/conf/distro/openbmc-phosphor.conf
+++ b/conf/distro/openbmc-phosphor.conf
@@ -56,3 +56,5 @@
 
 KERNEL_FEATURES_append = " phosphor-gpio-keys"
 KERNEL_FEATURES_remove_qemuall = " phosphor-gpio-keys"
+
+IMAGE_CLASSES_append = " image_types_phosphor"