This introduced an "advanced" menu, only for developers, and most
questions are grouped in there, because we cannot support users
playing with experimental features or work in progress (like the new
onewire code, or the new ptp daemon) until they are the officially
supported option.

The commit also removes changes in stack size, because the new
calibration code by tom is not stack-hungry like the previous one:
wrnic_defconfig is now the same as spec_defconfig and ppsi doesn't
select a bigger stack any more.

Finally, ppsi_defconfig replaces spec_ppsi_defconfig, since we only
support the "spec" hardware configuration (other implementations we know
of are still very much spec-alike).

Unfortunately, this spits two warnings:

    Kconfig:25:warning: defaults for choice values not supported
    Kconfig:29:warning: defaults for choice values not supported

even if such defaults actually works. The next commit removes them.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>