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/ximg.c b/cmd/ximg.c
index 770f6a3..73c7642 100644
--- a/cmd/ximg.c
+++ b/cmd/ximg.c
@@ -24,6 +24,7 @@
 #include <bzlib.h>
 #endif
 #include <asm/byteorder.h>
+#include <asm/cache.h>
 #include <asm/io.h>
 
 #ifndef CONFIG_SYS_XIMG_LEN