doc: genrest: Speed up documentation rebuilding

Skip writing index and symbol RST files whose content hasn't changed, to
avoid updating their timestamps. This makes documentation rebuilds much
faster, as Sphinx looks at the timestamp to determine if an RST file has
been updated.

Rebuilding docs with symbol reference up-to-date, before:

    $ time make html
    real    4m52.838s
    user    4m46.242s
    sys     0m4.249s

After:

    $ time make html
    real    0m48.731s
    user    0m47.571s
    sys     0m0.908s

Testing was done with 'make VERBOSE=1 SPHINXOPTS= html', suggested by
Marti Bolivar.

Piggyback a small cleanup to the code generating the select/imply
information.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>