Use current directory specifier in USE_MDFILE_AS_MAINPAGE
Use current directory specifier in USE_MDFILE_AS_MAINPAGE

For some reason (possibly a bug) Doxygen will use README from a
subdirectory (rather than product's root directory) as a main page, if it
finds one. Prefixing file name with `./` seems to prevent this.

michpolicht/CuteHMI,michpolicht/CuteHMI,michpolicht/CuteHMI,michpolicht/CuteHMI,michpolicht/CuteHMI