linux-aspeed: Fix defconfig SRC_URI

The defconfig in SRC_URI contains ast2400/2500 directory, when working
with devtool, it causes the defconfig to be copied into unexpected
subdirectory in working dir.

Fix it by specifying FILESEXTRAPATHS_prepend and remove the directory
from SRC_URI.

Resolves: openbmc/openbmc#3294
Tested: Build linux-apseed on both Palmetto and Romulus and verify the
        defconfig is correctly used.
        Build, clean, and rebuild linux-aspeed with devtool and verify
        the defconfig is correctly used.

Change-Id: I3ec9ab71f39f26e434f2eb593b27fbb34b69bd54
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/meta-openbmc-bsp/meta-aspeed/recipes-kernel/linux/linux-aspeed.inc b/meta-openbmc-bsp/meta-aspeed/recipes-kernel/linux/linux-aspeed.inc
index fcc40f1..9343393 100644
--- a/meta-openbmc-bsp/meta-aspeed/recipes-kernel/linux/linux-aspeed.inc
+++ b/meta-openbmc-bsp/meta-aspeed/recipes-kernel/linux/linux-aspeed.inc
@@ -8,8 +8,10 @@
 
 KSRC ?= "git://github.com/openbmc/linux;protocol=git;branch=${KBRANCH}"
 SRC_URI = "${KSRC}"
-SRC_URI_append_ast2400 = " file://ast2400/defconfig"
-SRC_URI_append_ast2500 = " file://ast2500/defconfig"
+SRC_URI += " file://defconfig"
+
+FILESEXTRAPATHS_prepend_ast2400 := "${THISDIR}/ast2400:"
+FILESEXTRAPATHS_prepend_ast2500 := "${THISDIR}/ast2500:"
 
 LINUX_VERSION_EXTENSION ?= "-${SRCREV}"