This makes life easier when calling e.g. `saul_reg_write()` with data
stored in flash.
As now the signatures for reading and writing differ (in that `const`
qualifier only), `saul_notsup()` is split into `saul_write_notsup()`
and `saul_read_notsup()`. However, one is implemented as a symbol alias
of the other, so that ROM consumption remains unchanged.
When the measurement results are read from the `ALG_RESULT_DATA` register set including the STATUS register, the `DATA_RDY` flag in the STATUS register is already cleared during reading. Therefore it is not possible to check this flag after the `ALG_RESULT_DATA` has been read. Therefore the function `ccs811_read_iaq` always returned `CCS811_ERROR_NO_NEW_DATA` although the data were valid either after checking for new data with the function `ccs811_data_ready` or after triggering the Data Ready interrupt.
The expandable GPIO API requires the comparison of structured GPIO types. This means that inline functions must be used instead of direct comparisons. For the migration process, drivers must first be changed so that they use the inline comparison functions.
