Move Aspeed kernel/U-Boot appends to ast2400 layer

Part 1 of 2 (meta-openbmc-bsp, meta-openbmc-machines)
diff --git a/meta-ast2400/recipes-kernel/linux/linux-obmc_%.bbappend b/meta-ast2400/recipes-kernel/linux/linux-obmc_%.bbappend
new file mode 100644
index 0000000..de6d3d6
--- /dev/null
+++ b/meta-ast2400/recipes-kernel/linux/linux-obmc_%.bbappend
@@ -0,0 +1 @@
+KERNEL_CONFIG_COMMAND = "oe_runmake_call -C ${S} O=${B} ${KMACHINE}_defconfig"