dropbear: allow more complex configuration

dropbear: allow more complex configuration

- switch DB_OPT_COMMON and DB_OPT_CONFIG to comma-separated lists:
  this allows to have values with "|" in DB_OPT_COMMON and DB_OPT_CONFIG
  which is more likely to be than values with commas;
  use $(comma) variable for values with commas.
- sort DB_OPT_COMMON and DB_OPT_CONFIG to have "overrides" on top of list.
- allow DB_OPT_COMMON to have values with commas.
- allow to replace multiline definitions in sysoptions.h.

improves e1bd9645

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>