Use out of (kernel) tree defconfig

I finally figured out how to get kernel config fragments working.
This allows removal of some earlier kernel recipe hacks.
Part 2 of 2 ( meta-phosphor, meta-openbmc-bsp )
diff --git a/meta-ast2400/recipes-kernel/linux/linux-obmc_%.bbappend b/meta-ast2400/recipes-kernel/linux/linux-obmc_%.bbappend
index de6d3d6..6d7ae27 100644
--- a/meta-ast2400/recipes-kernel/linux/linux-obmc_%.bbappend
+++ b/meta-ast2400/recipes-kernel/linux/linux-obmc_%.bbappend
@@ -1 +1,2 @@
-KERNEL_CONFIG_COMMAND = "oe_runmake_call -C ${S} O=${B} ${KMACHINE}_defconfig"
+FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
+SRC_URI += "file://defconfig"