initramfs-init: allow disabling auto serial tty
Automatically enabling detected serial consoles may collide with embedded platform which uses serial peripherals.
Disable the autodetection if:
- apkovl is used (previously detected login tty should be saved)
- boot option
autodetect=no
is set..
The autodetect boot option has not been used since the introduction of nlplug-findfs, in commit ba27888b (init: refactor to use nlplug-findfs).
fixes: #46 (closed) ref: aports#15265 ref: !135 (closed)