kselftest: devices: Allow specifying boards directory through parameter

Add support for a --boards-dir parameter through which the directory in
which the board files will be searched for can be specified. The
'boards' subdirectory is still used as default when the parameter is not
specified.

This allows more easily running the test with board files supplied by an
external repository like
https://github.com/kernelci/platform-test-parameters

.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Link: https://lore.kernel.org/r/20240613-kselftest-discoverable-probe-mt8195-kci-v1-1-7b396a9b032d@collabora.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>