start_getty: Over added SERIAL_CONSOLE cause error in userspace log

start_getty: Over added SERIAL_CONSOLE cause error in userspace log

Error log will be logged into /var/log/message.
Added in more condition checking on the script. Check
/proc/tty/drivers and /proc/tty/driver/*
file system to retrieve active targeted serial.
Only establish getty with active serial in runtime.

[YOCTO #10844]

Reviewed-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Choong YinThong <yin.thong.choong@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>