Move Aspeed kernel/U-Boot appends to ast2400 layer

Part 1 of 2 (meta-openbmc-bsp, meta-openbmc-machines)
diff --git a/meta-openbmc-bsp/meta-aspeed/meta-ast2400/recipes.txt b/meta-openbmc-bsp/meta-aspeed/meta-ast2400/recipes.txt
new file mode 100644
index 0000000..149509f
--- /dev/null
+++ b/meta-openbmc-bsp/meta-aspeed/meta-ast2400/recipes.txt
@@ -0,0 +1,2 @@
+recipes-bsp          - Anything with links to specific hardware or hardware configuration information
+recipes-kernel       - The kernel and generic applications/libraries with strong kernel dependencies