This makes the daemon less destructive at startup, especially if the ESP
is not mounted.
It's stored in 3 different places right now, so move it into one point of truth.
Now the ESP is detected when needed including all point of time safety checks and
dynamically mounted and unmounted if necessary.
This allows the ODM to specify command line arguments such as:
--flags=use-shim-unique,use-legacy-bootmgr-desc,no-ux-capsule
..which is useful for testing broken firmware.
Additionally, if the user specified something invalid, do not autodetect the
ESP but return with a journal error. It seems wrong to ignore what the user
explicitly set and perhaps do something dangerous.
Alternative to https://github.com/hughsie/fwupd/pull/599
