ui/ncurses: Improve update handling in nested screens

Several screens in petitboot-nc require an update if a config or sysinfo
update is received. However if those screens exist but are not the
current screen they will incorrectly try to draw to the screen.

Where the currently active screen is a textscreen (eg. a help screen)
the update is delayed until after the screen is exited.

In the particular case of nc-config where the current screen can be an
nc-subset screen, the nc-subset screen is exited immediately so the
update can be performed, since the nc-subset screen depends on the
information in the previous screen.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>