net: Move enetaddr env access code to env config instead of net config

In order that we can use eth_env_* even when CONFIG_NET isn't set, move
these functions to environment code from net code.

This fixes failures such as:

  board/ti/am335x/built-in.o: In function `board_late_init':
  board/ti/am335x/board.c:752: undefined reference to `eth_env_set_enetaddr'
  u-boot/board/ti/am335x/board.c:766: undefined reference to `eth_env_set_enetaddr'

which caters for use cases such as:

commit f411b5cca48f ("board: am335x: Always set eth/eth1addr environment
variable")

when Ethernet is required in Linux, but not U-Boot.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
diff --git a/board/gateworks/gw_ventana/gw_ventana.c b/board/gateworks/gw_ventana/gw_ventana.c
index 4ddc7e1..332c506 100644
--- a/board/gateworks/gw_ventana/gw_ventana.c
+++ b/board/gateworks/gw_ventana/gw_ventana.c
@@ -22,6 +22,7 @@
 #include <asm/setup.h>
 #include <dm.h>
 #include <dm/platform_data/serial_mxc.h>
+#include <environment.h>
 #include <hwconfig.h>
 #include <i2c.h>
 #include <fdt_support.h>