Portability: adding new secondary groups to a user account

[jimklimov]

Current solution relies on usermod -G $grpname -a $username (notably -a -- which does not exist in all system toolkits).

If time comes, fix this to use a new helper script with a more complex but portable solution to find all the secondary groups the user is a member of, and pass the standard usermod -G $grp,name,list $username command. Scripting elements can be found in setup/10-bios-gpio.sh scriptlet.