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/tcl/sl50/board.c b/board/tcl/sl50/board.c
index 7537fa2..afa6290 100644
--- a/board/tcl/sl50/board.c
+++ b/board/tcl/sl50/board.c
@@ -11,6 +11,7 @@
 #include <env.h>
 #include <errno.h>
 #include <init.h>
+#include <net.h>
 #include <serial.h>
 #include <spl.h>
 #include <asm/arch/cpu.h>