efi_loader: Run dhcp if an http boot option is selected

efi_loader: Run dhcp if an http boot option is selected

The EFI boot manager relies on having an IP address before trying to
boot an EFI HTTP(s) boot entry. However, defining it as a boot or
pre-boot command is not always the right answer since it will
unconditionally add delay to the board boot, even if we don't boot
over the network.

So let's do a DHCP request from the boot manager, if 'ipaddr' is
empty and fail early if we don't have an address.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>