env: net: U_BOOT_ENV_CALLBACKs should not depend on CMD_NET

Some environment variables are relevant for networking. For these
U_BOOT_ENV_CALLBACKs have been defined. When the corresponding environment
variable is updated the callback updates the state of the network
sub-system.

In the UEFI subsystem we can use the network even if CONFIG_CMD_NET is not
defined.

Let the usage of the U_BOOT_ENV_CALLBACKs depend on CONFIG_NET and not on
CONFIG_CMD_NET.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>