build rules: Identify build stage with simple variables

Provide simple environment variables telling which stage of boot is
being built. Also move this to arch-agnostic location.

Change-Id: I8cbb5cf91f53e01c06e7d672b5be3f5c235f911d
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5410
Reviewed-by: Aaron Durbin <adurbin@google.com>
Tested-by: build bot (Jenkins)

src/include/device/pci.h

@@ -19,7 +19,7 @@
 
 #include <stdint.h>
 #include <stddef.h>
-#include <arch/rules.h>
+#include <rules.h>
 #include <arch/io.h>
 #include <device/pci_def.h>
 #include <device/resource.h>