Create a phosphor-specific initramfs

OpenBMC has been using a .bbappend to add/remove packages from
core-image-minimal-initramfs.  Doing so keeps the recipe and initramfs
image name as core-image-minimal-initramfs which is misleading as it now
is very much phosphor-specific.  Instead, make a new
obmc-phosphor-initramfs recipe and image that is patterned after
core-image-minimal-initramfs.

Change-Id: Ieca2268eff01bb61cc0d25a1cbc1a6f5fadb6d2c
Signed-off-by: Rick Altherr <raltherr@google.com>
diff --git a/classes/image-overlay.bbclass b/classes/image-overlay.bbclass
index dbe4daf..68acca9 100644
--- a/classes/image-overlay.bbclass
+++ b/classes/image-overlay.bbclass
@@ -1,7 +1,7 @@
 PACKAGES = " "
 EXCLUDE_FROM_WORLD = "1"
 
-INITRD_IMAGE ?= "core-image-minimal-initramfs"
+INITRD_IMAGE ?= "obmc-phosphor-initramfs"
 INITRD ?= "${DEPLOY_DIR_IMAGE}/${INITRD_IMAGE}-${MACHINE}.cpio.${INITRD_CTYPE}${uboot}"
 
 IMAGE_BASETYPE ?= "squashfs-xz"