firstboot: some love (#39070)

This is split out of #38764.

It mostly introduces the "chrome" stuff that puts a blue at the top of
bottom of the terminal screen when going through interactive tools such
as firstboot, homed-firstboot, and (in future) systemd-sysinstall).

it also introduces a generic "prompt_loop()" helper thatn queries the
user for some option in a loop until the rsponse matches certain
requirements. It's a generalization of a function of the same name that
so far only existed in firstboot.c. The more generic version will be
reused in a later PR by homed-firstboot and by sysinstall.