Add override for aspeed bsp machine.

Upstream yocto is organized such that there is only bblayer for all
builds and overrides are used to customize recipes for any specific
platform.

It's possible for a group to use openbmc/openbmc as part of their
distro and run into issues with recipes that are comingled.

Change-Id: I5dc6e8c1448d95da1b5ee64b220e2955466cdf4e
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/meta-ast2400/recipes-bsp/u-boot/u-boot_%.bbappend b/meta-ast2400/recipes-bsp/u-boot/u-boot_%.bbappend
index ef6a709..e3cc9e2 100644
--- a/meta-ast2400/recipes-bsp/u-boot/u-boot_%.bbappend
+++ b/meta-ast2400/recipes-bsp/u-boot/u-boot_%.bbappend
@@ -1,3 +1,3 @@
-FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
+FILESEXTRAPATHS_prepend_ast2400 := "${THISDIR}/files:"
 
-SRC_URI += "file://fw_env.config"
+SRC_URI_append_ast2400 = " file://fw_env.config"