rime/collect.c: fix set-but-unused variable

Use dependent on debug infrastructure.

Supports pabigot/contikix#6
rime/collect-neighbor.c: remove set-but-unused

Supports pabigot/contikix#6
apps/powertrace: fix set-but-unused variable

All fields are extracted but only a few used.  Add void references in lieu
of removing the unused values, to simplify temporary adaptation of the
output.

Supports pabigot/contikix#6
apps/shell: work around set-but-unused variable

Saved but unchecked return value is probably a bug.

Supports pabigot/contikix#6
apps/shell: work around set-but-unused variable

Assumes this static variable is present for inspection under a debugger.
Make the declaration volatile to ensure the value is available; still also
need the bogus reference to avoid the warning that it is unused.

Supports pabigot/contikix#6
apps/shell: remove unused code (and variable)

Code disabled by in ce53d7b748c three years ago.

Supports pabigot/contikix#6
cc2420: fix set-but-unused variable

API requires a variable into which a useless value is read; add a void
reference to eliminate the resulting warning

Supports pabigot/contikix#6
cc2420: eliminate unused variable

Supports pabigot/contikix#6
cc2420-arch-sfd: eliminate unnecessary declaration

Read of volatile peripheral register can be achieved without storing it in a
variable.

Supports pabigot/contikix#6
NO UPSTREAM MERGE sht11: eliminate warning

This code uses #warning to notify the user that the driver unilaterally
disables the primary I2C peripheral in order to bit-bang the non-standard
protocol required.  The warning might even be useful, if it wasn't lost in
other warnings.  To make it possible to find those other warnings, this
commit disables this warning.  Make sense?

Probably no more so than having a driver disable a peripheral bus and not
provide a facility to re-enable it again.

Supports pabigot/contikix#6