To make the debug output of the database more readable
the has_cal_data lookup for RC data prints the file that
is looked up similar to the lookup for file system.
This adds the possibility to read cal data from flash/EEPROM by adding
callbacks to the database. Unlike the RC and FILESYSTEM data, this is
very device-specific, but we can let devices register callbacks in the
database so that reading cal data from flash can use the same APIs as
from RC or filesystem.
Note that this also gives a convenient way to inject call data during
unit tests, if desired.
This is a refactoring with no functional change. Instead of hard-coding
the lookup of RC and FILESYSTEM data, we loop over a structure holding
those. This will make it easier to add more types of data lookup in the
future.
This class contains methods to store and retrieve data from the local
calibration database. Note that in this case, the "database" is just a
bunch of files on the local filesystem.
