Newer versions of dtc agressively pack the string table, using *any* matching
suffix rather than whole strings. This is specification compliant, but breaks
if you use the offset as a hash key as a [premature] optimization.
Just use the string table as a flat block of memory rather than pre-parsing it.
This fixes reading the system FDT on new OpenBMC builds.
Some parsers are ignoring the magic when using _FLAG_IGNORE_CHECKSUM
(which is wrong; fuzzers have no problem with enforcing a static prefix)
and other either disregard the offset or check the magic in an unsafe
way. Also, use FWUPD_ERROR_INVALID_FILE consistently for magic failure.
Add a vfunc, and move all the clever code into one place.
