boot: Improve device_path_to_str_internal()

boot: Improve device_path_to_str_internal()

The UEFI spec has a generic `Path` node representation that can be used
for device path nodes that are unknown. So we can use that instead of
giving up when we see a node other than FilePath.

This also simplifies the FilePath case by just using xasprintf(). The
code is really just a fallback for silly firmware that does not
implement EFI_DEVICE_PATH_TO_TEXT_PROTOCOL (looking at you, Apple).

The correctness of this was tested by round-tripping it through
EFI_DEVICE_PATH_FROM_TEXT_PROTOCOL, which yielded an identical device
compared to our input path.