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/board/amlogic/sei610/sei610.c b/board/amlogic/sei610/sei610.c
index b17eb9e..5c24276 100644
--- a/board/amlogic/sei610/sei610.c
+++ b/board/amlogic/sei610/sei610.c
@@ -7,6 +7,7 @@
 #include <common.h>
 #include <dm.h>
 #include <env_internal.h>
+#include <net.h>
 #include <asm/io.h>
 #include <asm/arch/axg.h>
 #include <asm/arch/sm.h>