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:
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