common: generate legacy_config_opts.h from .yaml.in files

common: generate legacy_config_opts.h from .yaml.in files

* add a setting named "with_legacy" to .yaml.in files, so
  each option with a true "with_legacy" will have an entry
  in legacy_config_opts.h.
* preserve the comments from legacy_config_opts.h to .yaml.in,
  some of them are solely for developers, but some of them are
  good reading for users as well. we can use them for "desc"
  field in a follow-up change.
* move common/legacy_config_opts.h to common/options/legacy_config_opts.h
  as legacy_config_opts.h is "closer" to the options directory
  than other sources files under src/common.
* update y2c.py to generate separate .h files which are in turn
  included by legacy_config_opts.h
* add a target named "legacy-option-headers", and let
  some targets depend on it so that these headers generated by
  y2c.py can be generated before the .cc files including them
  are compiled.

Signed-off-by: Kefu Chai <kchai@redhat.com>