Introduce FLASHFILE variable to start migrating boards to use it.
This is the file that will be used for flashing.
Boards do not currently use it but will migrated in upcoming PRs.
While info-build already provides lots of useful information
it does not print the application source dir. However this
might be useful for debugging and logging, hence its added
to the output.
Add a target to print an internal variable value.
Usage: make info-debug-variable-VARIABLENAME
Example:
make info-debug-variable-ELFFILE
/path/to/riot/examples/hello-world/bin/native/hello-world.elf
