Move fdt handling logic out of main.c.

This is in preparation for having archs/platforms that don't use FDT.
diff --git a/inc/cpu.h b/inc/cpu.h
index 58df21d..30433ad 100644
--- a/inc/cpu.h
+++ b/inc/cpu.h
@@ -17,11 +17,11 @@
 
 /* TODO: Update alignment such that cpus are in different cache lines. */
 struct cpu {
-	struct vcpu *current;
-
 	/* CPU identifier. Doesn't have to be contiguous. */
 	size_t id;
 
+	struct vcpu *current;
+
 	/* Pointer to bottom of the stack. */
 	void *stack_bottom;