scripts: get_abi.pl: fix parse logic for DT firmware

scripts: get_abi.pl: fix parse logic for DT firmware

It doesn't make any sense to parse ABI entries under
/sys/firmware, as those are either specified by ACPI specs
or by Documentation/devicetree.

The current logic to ignore firmware entries is incomplete,
as it ignores just the relative name of the file, and not
its absolute name. This cause errors while parsing the
symlinks.

So, rewrite the logic for it to do a better job.

Tested with both x86 and arm64 (HiKey970) systems.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1c806eaec96f6706db4b041bbe6a0e2519e9637e.1632750315.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>