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.
SAUL read functionality implemented only, as described in
tsl4531x_saul.c.
This driver will work with SAUL in both low and high power modes,
with the startup mode configurable in tsl4531x_params.h.
Changes include:
- Implementation of two power modes for the driver: low and high
- Redesign and API change to the description in tsl4531x.h
- Full documentation
- Changing file structure and implementation to fit best practices as
described in
https://github.com/RIOT-OS/RIOT/wiki/Guide%3A-Writing-a-device-driver-in-RIOT
- Including I2C addresses for the rest of the range
