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.
This changes the prefixes of the symbols generated from USEMODULE and
USEPKG variables. The changes are as follow:
KCONFIG_MODULE_ => KCONFIG_USEMODULE_
KCONFIG_PKG_ => KCONFIG_USEPKG_
MODULE_ => USEMODULE_
PKG_ => USEPKG_
A memset was used to clear two of the SAUL values in phydat_t::val
The change is to replace that by the following
res->val[1] = 0;
res->val[2] = 0;
which makes the code more obvious.
Problem:
Since `i2c_read_reg` returns (signed) `int` values, the result can
be negative. However, the variable used to save the result is unsigned.
Fix:
Add and use a signed variable for return value storing.
