common: Move random-number functions into their own header

Create a new rand.h header file and move functions into it, to reduce
the size of common.h

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

include/net.h

@@ -16,6 +16,7 @@
 #include <asm/byteorder.h>	/* for nton* / ntoh* stuff */
 #include <env.h>
 #include <linux/if_ether.h>
+#include <rand.h>
 
 #define DEBUG_LL_STATE 0	/* Link local state machine changes */
 #define DEBUG_DEV_PKT 0		/* Packets or info directed to the device */