UBUNTU: [Packaging] scripts/misc/kernelconfig: Disable config checks for mainline...

UBUNTU: [Packaging] scripts/misc/kernelconfig: Disable config checks for mainline builds

For mainline builds, all checks need to be disabled since failures can't
be tolerated/handled. For that, do_skip_checks is set to 'true' but the
new kernelconfig script doesn't honor that. Fix it.

While at it, add a header comment listing all supported environment
variables.

Signed-off-by: Juerg Haefliger <juerg.haefliger@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>