migrate Drone CI workflow to GitHub Actions

add Drone CI configuration to build firmware

extract button processing and coffee/cleaning routines into functions

define coffee flags as pre-processor macros Right now we need at least some knowledge about the values 0-4 representing five possible states of coffee making. Make it readable without this knowledge.

minor code cleanup

Replaced 5 step if-else by switch-case

Switched to size optimization

Bundled boolean flags into single 8bit mask

Copyright + License; reformatting

Separated Makefile and config; added build instructions