This allows several functions to be static in the files where they
are defined. See temperature.c for an example.

But mainly, the idea is allowing ease addition of features (likely
Kconfig'd ones) for specific WR users, without any modification to the
main function and other generic code.  This will happen soon for snmp,
for example.

We may push this a little further, by moving all shell stuff into
shell.c, the ptp task within ppsi itself, and so on. But now my time
is over.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>