boards/pic32-xx: set FLASHFILE to HEXFILE

[cladmi]

Contribution description

The boards are using HEXFILE for flashing even if there is no
FLASHER for the moment.

Testing procedure

The board do not have any flasher yet.
But in #9259 the flasher was added for pic32-wifire and pic32prog requires the HEXFILE.

Testing without board

The hexfile is used as flashfile and it can be overwritten from the environment.

BOARD=pic32-wifire make --no-print-directory -C examples/hello-world/ info-debug-variable-FLASHFILE
/home/harter/work/git/RIOT/examples/hello-world/bin/pic32-wifire/hello-world.hex
BOARD=pic32-clicker make --no-print-directory -C examples/hello-world/ info-debug-variable-FLASHFILE
/home/harter/work/git/RIOT/examples/hello-world/bin/pic32-clicker/hello-world.hex

FLASHFILE=lala BOARD=pic32-wifire make --no-print-directory -C examples/hello-world/ info-debug-variable-FLASHFILE
lala
FLASHFILE=lala BOARD=pic32-clicker make --no-print-directory -C examples/hello-world/ info-debug-variable-FLASHFILE
lala

Issues/PRs references

• Part of the FLASHFILE introduction Makefiles: add support to generate both .hex and .bin file and add FLASHFILE variable #8838
• part of pic32-wifire: add support for flashing with pic32prog pic32-wifire: add support for flashing with pic32prog #9259

[MrKevinWeiss]

It seems OK. Since there is no flash I cannot test on a board. If a flasher is added we should ensure it used FLASHFILE insead of the HEXFILE variable I suppose.

ACK!

[cladmi]

Thank you for the review.