efi_loader: provide function to get last node of a device path

On a block device and its partitions the same protocols can be
installed. To tell the apart we can use the type of the last
node of the device path which is not the end node.

The patch provides a utility function to find this last node.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>