build a set of flags for the different enabled subsystems

build a set of flags for the different enabled subsystems
From: Daniel Lezcano <dlezcano@fr.ibm.com>

When we want to check if a subsystem is enabled, we look at the
presence of a file/directory in the configuration tree files. That
works until we chroot into the rootfs. Some subsystem should be
preferably setup after the chrootfs, making the code simpler and
easier to read. So before setup the different subsystem, I build a
flags set and reuse it later to check if the subsystem is enabled or
not.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>