nic: Fix incorrect wrap handling of RX circular buffer

nic: move other nic functions into the dedicated nic files
uip: alloc buffer multiple of 8 bytes

Signed-off-by: João Silva <jgc3silva@gmail.com>

test/test_httpd_tx.c

@@ -112,7 +112,7 @@ void read_reg_timer(uint32_t *tmr) { *tmr = 0; }
 
 /* uip-conf.h asks uIP not to define the packet buffer: on the switch it lives
  * at a fixed XDATA address, so the bench provides the storage itself. */
-u8_t uip_buf[UIP_BUFSIZE + 2];
+u8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 
 uint16_t strlen_x(const char *s) { return (uint16_t)strlen(s); }