boards/native: define FLASHFILE

[cladmi]

Contribution description

Define FLASHFILE for native.

Even if native does not require flashing, it is the file used for running.

Testing procedure

The variable is set correctly and can be overwritten from the environment.

make --no-print-directory -C examples/hello-world/ info-debug-variable-FLASHFILE
/home/harter/work/git/RIOT/examples/hello-world/bin/native/hello-world.elf

FLASHFILE=last_flashfile_missing make --no-print-directory -C examples/hello-world/ info-debug-variable-FLASHFILE
last_flashfile_missing

Issues/PRs references

Part of introducing FLASHFILE #8838

[cladmi]

No need to put CI: run test tests are run on native anyway.

[MrKevinWeiss]

ACK. Tested on native with FLASHFILE=/home/kevinweiss/WorkingDirectory/RIOT/examples/hello-world/bin/native/hello-world.elf make flash term -C tests/shell/
hello_world appears not the shell so flashfile was overridden.

[cladmi]

Thank you for the review.

You are right, I should have put the execution test as I modified TERMPROG.
I wrote the testing procedure before updating it during the redaction.