Signed staging modules should not fail the taint check

[Juerg Haefliger]

ubuntu_boot/kernel_taint_test.py currently fails if staging modules are loaded since that taints the kernel. We intentionally sign a handful of modules from the staging directory which are deemed safe so if any of them is loaded it should not result in a taint test failure (due to the 'staging driver was loaded' taint flag).

[Po-Hsu Lin (cypressyew)]

Benefit for this:
* We don't need to maintain a hard-coded whitelist in our test.

Potential issue for this:
* Is it possible to have a staging module signed accidentally?
* Inconsistency for K-raspi test result on different devices, the bcm2835_codec module is unsigned and in staging with rpi4b8g, however it's just reported as in staging with rpi3bp
* Some kernel will still need a white list (e.g. B-5.4 raspi, in which those modules are in staging and unsigned), unless we want to hint the test case