[SRU][G/aws][PATCH 0/1] fix broken /dev/console on i3.metal

[Andrea Righi]

BugLink: https://bugs.launchpad.net/bugs/1896604

[Impact]

Starting with kernel 5.8 the default nr_uarts has been changed from 4 to
2 for amd64, but this seems to affect i3.metal instances in AWS, because
ttyS0 is now remapped to ttyS4 and this is breaking tools like
cloud-init (and probably something else).

[Test case]

# echo > /dev/console
bash: echo: write error: Input/output error

[Fix]

Setting nr_uarts=4 by default (via CONFIG_SERIAL_8250_RUNTIME_UARTS)
restores the previous behavior and writing to /dev/console works without
returning any error.

[Regression potential]

Minimal. Restores the old behavior used in 5.4 (that shouldn't have
changed in the first place).