u-boot-fw-utils: Fix mtd-paritions warning

u-boot-fw-utils_%.bbappend matches the upstream Yocto u-boot
recipe in addition to our own, generating a warning when
the upstream recipe is parsed because the mtd partition
rules are not in the default FILES search path.  Add our
layer path to the search path.

Tested: Built and verified mtd-paritions rule file in rootfs
Change-Id: I1e3d769d0066620a626c3be6f9f410ef4476fc66
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-phosphor/common/recipes-bsp/u-boot/u-boot-fw-utils_%.bbappend b/meta-phosphor/common/recipes-bsp/u-boot/u-boot-fw-utils_%.bbappend
index a460f40..6f9c5e6 100644
--- a/meta-phosphor/common/recipes-bsp/u-boot/u-boot-fw-utils_%.bbappend
+++ b/meta-phosphor/common/recipes-bsp/u-boot/u-boot-fw-utils_%.bbappend
@@ -1,3 +1,4 @@
+FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
 SRC_URI += "file://76-mtd-partitions.rules"
 
 do_install_append() {