efi_loader: add efi_dp_from_name()

Factor out efi_set_bootdev() and extract efi_dp_from_name().
This function will be used to set a boot device in efishell command.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>