dropbear: introduce config option "keyfile" (replacement for "rsakeyfile")

dropbear: introduce config option "keyfile" (replacement for "rsakeyfile")

* option "keyfile" is more generic than "rsakeyfile".
* option "rsakeyfile" is considered to be deprecated and should be removed
  in future releases.
* warn user (in syslog) if option "rsakeyfile" is used
* better check options ("rsakeyfile" and "keyfile"): don't append
  "-r keyfile" to command line if file is absent (doesn't exist or empty),
  warn user (in syslog) about such files

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