efi_loader: ACPI device node to text

The device path to text protocol renders ACPI device nodes incorrectly.

Use capital hexadecimal numbers as shown in the UEFI spec examples.

Always output the optional UID. This matches what UEFI SCT expects and
saves us an `if`.

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