Stable: Integration with CI continues

Changes:

• Few small changes in the way mbedgt returns codes in CI jobs
  • Job will fail if at least one test fails: return code will be >0
  • Job fails if no specified tests were found for given target->platform: return code will be <-10
  • Job fails if no devices were specified for given targets: return code will be <-100
• Added Doxygen style documentation for existing APIs