systemd-automount will unmount the ESP when not in use for some
people. This causes automatic ESP detection to fail.
In this case the ESP will need to be added to the conf file and
then this commit will let it keep working.
/boot is a special cased directory when using ProtectSystem=full
Due to this, it's marked read only even if it's listed in ReadWritePaths.
Allow folks to use this for their ESP, but they need to create /boot/EFI
in advance of starting fwupd.
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
