obmc-phosphor-initramfs: Add read-only-rootfs feature

This reduces the number of files that end up in the image like the opkg
alternatives files that only get removed if the image class sees the
image featurs are set to read-only.

Change-Id: Icc93bdc25447b469693c26d286eb23d92978513c
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/recipes-phosphor/images/obmc-phosphor-initramfs.bb b/recipes-phosphor/images/obmc-phosphor-initramfs.bb
index 3f0b3c1..739aab5 100644
--- a/recipes-phosphor/images/obmc-phosphor-initramfs.bb
+++ b/recipes-phosphor/images/obmc-phosphor-initramfs.bb
@@ -5,7 +5,7 @@
 PACKAGE_INSTALL = "${VIRTUAL-RUNTIME_base-utils} base-passwd ${ROOTFS_BOOTSTRAP_INSTALL} obmc-phosphor-initfs"
 
 # Do not pollute the initrd image with rootfs features
-IMAGE_FEATURES = ""
+IMAGE_FEATURES = "read-only-rootfs"
 
 export IMAGE_BASENAME = "obmc-phosphor-initramfs"
 IMAGE_LINGUAS = ""