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.
The Silicon Labs Si705x sensors (Si7050/1/3/4/5) are very similar to the
Si7021 sensors featuring only a temperature sensor and no humidity
sensor. The only difference between the Si705x is the temperature
accuracy of the reading, ranging from +/- 0.1 C in the Si7051 to +/- 1 C
in the Si7050.
This patch adds support for this family of sensors extending the
functionality of the existing si70xx driver. Following the style of
other modules, this implements a pseudomodule per supported chip, adding
si7050, si7051, si7053, si7054 and si7055 pseudomodules.
As a minor change this patch also implements the missing
si70xx_get_serial, si70xx_get_id and si70xx_get_revision functions that
were declared in the si70xx.h header but implemented as private
functions. The si70xx_get_id() may be relevant for the application to
know at run time exactly which version of the hardware is installed.
The updated test running with a Si7051 shows the following output, which
seems consistent with the room temperature conditions during the test.
```
make SI70XX_VARIANT=si7051 -C tests/driver_si70xx/ all flash
```
```
SI70XX temperature and humidity sensor test application
Initializing sensor...
[OK]
Found SI7051 sensor, revision 32
temperature: 24.71 C
temperature: 24.69 C
```
