[efi] Record cached DHCPACK from loaded image's device handle, if present

Record the cached DHCPACK obtained from the EFI_PXE_BASE_CODE_PROTOCOL
instance installed on the loaded image's device handle, if present.

This allows a chainloaded UEFI iPXE to reuse the IPv4 address and DHCP
options previously obtained by the built-in PXE stack, as is already
done for a chainloaded BIOS iPXE.

Signed-off-by: Michael Brown <mcb30@ipxe.org>