Remove FNGS logo from defconfigs and update scripts to load external logo for FNGS boot

This is a part of the fix for issue YT-162 "FNG System Logo: Replace in kernel logo with runtime logo or similar"

In this MR:

• removed a kernel config option in charge of FNG System logo from defconfigs;
• removed a switching between FNG System and Yocto logos in linux-seconorth.inc;
• updated bootscript-uboot-alt to load the previously provided logo file, if available;
• moved a memory address for ramdisk to lower address in bootscript-alt to avoid conflicts with bootlogo, address for which is already hardcoded in many other places.

Other MRs:

• GUF kernel MR: seco-ne/kernel/linux-guf!320 (merged)
• KUK kernel MR: seco-ne/kernel/linux-imx-kuk!533 (merged)
• meta-seconorth-distro MR: meta-seconorth-distro!788 (merged)