genrandconfig: verbose output and use stderr

genrandconfig: verbose output and use stderr

The output of genrandconfig is currently very terse, which is annoying
for debugging the script or generally seeing what is going on. Also the
timing information added by log_write isn't very useful when the script
is used stand-alone.

In the new setup, (verbose) output goes to stdout and error output goes
to stderr. Also the "INFO: generate the configuration" message is
eliminated - it should go in the autobuild-run script.

We also add an explicit message when a toolchain can't be used after
the first defconfig, otherwise autobuild-run will just silently
restart.

Note that, since the output of make is no longer redirected to
/dev/null, we get one more message on stderr that will be recorded in
the autobuilder's log file: KCONFIG_SEED=0xXXXXXXXX.

This approach allows us to optimise the error handling to use
exceptions, where appropriate, which can be caught at the top level and
converted to an error message to stderr. This, in turn, allows us to use
subprocess.check_call, which eliminates a lot of conditions.

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