phosphor: obmc-bsp-common: use stronger assigments

Make stronger assertions about the initramfs images and initramfs image
types to use.  Upstream BSP layers may set defaults for these variables
using ?= resulting in the variable settings here being ignored.

(From meta-phosphor rev: 2f24647373e7ecee0afafdbf9e84b6e883422bfe)

Change-Id: I27c316341690ffc3b85f19978aa811ebf84293cf
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-phosphor/conf/machine/include/obmc-bsp-common.inc b/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
index 7162b5d..2026260 100644
--- a/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
+++ b/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
@@ -11,6 +11,6 @@
         'mtd-static mtd-static-tar mtd-static-alltar', d)}"
 
 INITRAMFS_CTYPE ?= "lzma"
-INITRAMFS_FSTYPES += "cpio.${INITRAMFS_CTYPE}"
-INITRAMFS_IMAGE ?= "obmc-phosphor-initramfs"
-INITRAMFS_IMAGE_df-obmc-ubi-fs ?= ""
+INITRAMFS_FSTYPES = "cpio.${INITRAMFS_CTYPE}"
+INITRAMFS_IMAGE = "obmc-phosphor-initramfs"
+INITRAMFS_IMAGE_df-obmc-ubi-fs = ""