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_reg_rm can not be used safely unless in tight coordination with the
application. This points out the issue, puts the onus of making sure
nobody calls vanishing registrations on the device driver and deprecates
the call (as the requirements for correct usage are practically
impossible to fulfull for a generic driver).
