verify CRC before downloading and programming application

  Verify if the programmed application has the same 16-bit CRC as a seperate CRC-file on the TFTP-server.
  
  There are two modes possible:
  * Test if CRC-file exists.
    - If yes, then immediately start application.
    - if no, then try to load and program application
    
  * Verify content of crc file.
    - If file exists and content matches current application CRC, then immediately start application.
    - If file does not exist or content does not match current application CRC, then try to load and program application
  
  The filename of the CRC-file is constructed out of the application filename having additional formatting keys.
  %m : replace with '-[MAC-ADDRESS]' (13 characters hex uppercase)
  %c : replace with '-[APPCRC]' (5 characters hex uppercase)
  %e : replace with '.crc' - the rest of the filename is truncated
  %C : replace with '' - verify CRC-file content
  
  Example:
  Supplied filename:    ethersex%m%c%e.bin
  CRC-filename:         ethersex-DEADBEEFCAFE-1337.crc
  Application filename: ethersex.bin