buildroot

genrandconfig: calculate configfile only once

The path to the .config file is calculated in several places - replace
it with a single calculation, and pass configfile as an argument
to is_toolchain_usable and fixup_config. These functions also don't
need outputdir any more.

This makes it easier to fix the case when configfile is not in
outputdir.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>