common: Drop net.h from common header

Move this header out of the common header. Network support is used in
quite a few places but it still does not warrant blanket inclusion.

Note that this net.h header itself has quite a lot in it. It could be
split into the driver-mode support, functions, structures, checksumming,
etc.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/mem.c b/cmd/mem.c
index 18f0510..24a9c77 100644
--- a/cmd/mem.c
+++ b/cmd/mem.c
@@ -19,6 +19,7 @@
 #include <flash.h>
 #include <hash.h>
 #include <mapmem.h>
+#include <rand.h>
 #include <watchdog.h>
 #include <asm/io.h>
 #include <linux/compiler.h>