We don't actually need either of the things it provides (looking up in source
and built, and converting to an absolute path) so just replace it with
g_build_filename() instead.
This also has the advantage that it does the right thing on Windows.
This allows hardware from OEMs to *not* match generic firmware supplied by the
device manufacturer. The idea being, that the OEM will supply firmware that
will actually work on the device.
Based on a patch from Mario Limonciello, many thanks.
