efi_loader: export efi_locate_device_handle()

This function will be used in the next commit where some behavior
of EFI boot manager will be expanded.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>