efi_loader: Let networking support depend on NETDEVICES

CONFIG_NET does not imply that there are actually network devices
available, only CONFIG_NETDEVICES does. Changing to this dependency
obsoletes the check in Kconfig because NETDEVICES means DM_ETH.

Fixes: 0efe1bcf ("efi_loader: Add network access support")
Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>