fng-install: make it possible to define a background color during boot via a new command-line argument

This is a fix for https://secogroup.atlassian.net/browse/YT-130 "bootlogo: fng-install: Pass background parameter from install script to the post-install step"

In this MR:

• added a "--BackgroundColor" command line argument to fng-install script. When specified, a provided value is saved to /boot/logo_background_color file;
• reworked prepare-logo.sh to extract a background color from /boot/logo_background_color, if background color is already not defined (e.g. via "-b" input argument for prepare-logo.sh).