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>

arch/x86/lib/coreboot_table.c

@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <malloc.h>
+#include <net.h>
 #include <vbe.h>
 #include <acpi/acpi_s3.h>
 #include <asm/coreboot_tables.h>