z50-raspi-firmware: Fix serial console output sent to bluetooth port with kernel 5.x (Closes: #941974)
This fixes wrong used serial output. The code for estimating the version
of the kernel is buggy and no longer work with kernel 5.x. In such a
case, the test that switch to use "ttyS1" is not used, which defaults
to ttyAMA0. Thanks to Thorsten Glaser for the fix !