krn-src: use lsb_release to autodetect DIST

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
Merge pull request #1 from RobertCNelson/src-dist-dpkg

krn-src: use lsb_release to autodetect DIST
enable-spi: determine unused phandle rather than guessing + properly escape spi dt node name.
get-rcn-kernel-source: DIST can be set manually or inferred automatically, if possible.
documentation update.