PREFLASHER/PREFFLAGS/FLASHDEPS are evaluated by the main Makefile.include.
Their value does not need to be exported.
Testing
-------
`git diff --word-diff` only reports `export` being removed.
`git show --stat` reports `16 insertions(+), 16 deletions(-)`
Which is the same amount as lines that where matching
`export[[:blank::]]\+VARIABLE` plus the newline that is said to have
changed.
79280eb12e