efi_loader: Uart device path

When uploading an EFI binary via the UART we need to assign a device path.

* Provide devicepath node to text conversion for Uart() node.
* Provide function to create Uart() device path.
* Add UART support to efi_dp_from_name().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>