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 f411b5cc

 ("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>