meta-phosphor: Ensure INITRAMFS_CTYPE defined

The hardware platforms use obmc-bsp-common.inc, which defines
INITRAMFS_CTYPE, but the simulation (qemu) platforms do not.
core-image-minimal-initramfs.bbappend currrently uses INITRAMFS_CTYPE
so we need to make sure it is defined.  Default there to 'lzma' if not
already defined.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/common/recipes-core/images/core-image-minimal-initramfs.bbappend b/common/recipes-core/images/core-image-minimal-initramfs.bbappend
index 90a1c3f..5710d22 100644
--- a/common/recipes-core/images/core-image-minimal-initramfs.bbappend
+++ b/common/recipes-core/images/core-image-minimal-initramfs.bbappend
@@ -1,3 +1,4 @@
 PACKAGE_INSTALL_remove = "initramfs-live-boot initramfs-live-install initramfs-live-install-efi"
 PACKAGE_INSTALL += "obmc-phosphor-initfs"
+INITRAMFS_CTYPE ?= "lzma"
 INITRAMFS_FSTYPES = "cpio.${INITRAMFS_CTYPE}"