[arch] move the atomic ops into a separate header

Now you need to include arch/atomic.h to get to the atomic routines.
This simplifies a recusion issue in the way arch/ops.h included
arch_ops. Also just generally makes things cleaner.

external/lib/lwip/netif.c

@@ -1,5 +1,6 @@
 #include <dev/class/netif.h>
 #include <kernel/event.h>
+#include <arch/atomic.h>
 #include <arch/ops.h>
 #include <netif/etharp.h>
 #include <lwip/netif.h>