While combining all configuration data into /etc/usb-moded/usb-moded.ini
works as expected when new configuration files are installed to the device,
it also means that 1) uninstalling / updating configuration files has no
effect and 2) /etc can't reside on a read-only file-system.
Maintain "delta to static configuration" and store it outside /etc - in
/var/lib/usb-moded/usb-moded.ini, so that also changes in / removal of
existing configuration files are handled in deterministic manner.
If /etc/usb-moded/usb-moded.ini exists, migrate data from it and then remove
the now unnecessary file.
Streamline handling of non-existing configuration and remove functions made
redundant in the process.
Note: The "read from filesystem on each setting access" is retained for now
to avoid regression is situations where such behavior might be expected.
Signed-off-by: Simo Piiroinen <email@example.com>