This allows including the header without using the module. Obviously,
calls to the functions provided by the header won't like without using
the module. But including the header can still be useful for e.g.:
if (IS_USED(MODULE_PCF857x)) {
/* make use of the module */
}
In the above example all calls to pcf857x functions would be optimized
out when the module is not used, full compile checks happen in either
case.
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.
