env: Move non-cli env functions to env/common.c

Move the following functions from cmd/nvedit.c to env/common.c:
  env_set_ulong()
  env_set_hex()
  env_get_hex()
  eth_env_get_enetaddr()
  eth_env_set_enetaddr()
  env_get()
  from_env()
  env_get_f()
  env_get_ulong()
since these functions are not specific for U-Boot's CLI.

We leave env_set() in cmd/nvedit.c, since it calls _do_env_set(), which
is a static function in that file.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>