[cli] support default config without specifying a config file

[wgtdkp]

This PR includes two small enhancements:

1. Support starting the commissioner-cli without specifying a configuration file. a. pskc is set to all-one. b. log messages are printed to syslog (/var/log/syslog).
2. Add sys logger support.

[codecov-commenter]

Codecov Report

Merging #203 (d96b74d) into main (ae90be0) will increase coverage by 0.13%. The diff coverage is 76.47%.

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
+ Coverage   68.76%   68.90%   +0.13%     
==========================================
  Files          52       55       +3     
  Lines        4870     4933      +63     
==========================================
+ Hits         3349     3399      +50     
- Misses       1521     1534      +13     

Impacted Files	Coverage Δ
src/app/cli/interpreter.hpp	100.00% <ø> (ø)
src/app/logger_util.hpp	54.16% <54.16%> (ø)
src/app/sys_logger.cpp	68.57% <68.57%> (ø)
src/app/cli/interpreter.cpp	54.21% <93.10%> (+1.69%)	:arrow_up:
src/app/cli/main.cpp	94.11% <100.00%> (+0.36%)	:arrow_up:
src/app/file_logger.cpp	90.32% <100.00%> (+15.77%)	:arrow_up:
src/app/sys_logger.hpp	100.00% <100.00%> (ø)