examples: ota_update: make some parameters changeable by environment variable
examples: ota_update: make some parameters changeable by environment variable

jbeyerstedt/RIOT-OTA-update,jbeyerstedt/RIOT-OTA-update,jbeyerstedt/RIOT-OTA-update,jbeyerstedt/RIOT-OTA-update,jbeyerstedt/RIOT-OTA-update