Add fngsystem-console-update.sh to the fngs image

This is a part of fix for issue [746-000797] Flash-N-Go System: Implement "Disable serial console".

This MR introduces following changes:

• enables ttynull console in kernel configuration for different boards, so it's possible to activate it in kernel cmdlline via scripts;
• adds fngsystem-console-update.sh from previous MR to the fngsystem-image.

Two other related MRs:

• seco-ne/tools/yocto-install!34 (merged)
• meta-seconorth-distro!719 (merged)