Makefile.include: do not build HEXFILE by default anymore

[cladmi]

Contribution description

All boards must now use FLASHFILE

Testing procedure

Building and testing in CI will work as expected.
I will only fail due to the waiting for other PR.

Solves the issue for mips-malta

This change has been dropped as board/mips-malta has been removed.

boards/mips-malta: do not overwrite HEXFILE

Building the .hex file is failing on mips-malta.
Now that HEXFILE is not always built, it is not necessary to overwrite
it with BINFILE.

When only removing the HEXFILE = $(BINFILE) you get the following error.

DOCKER="sudo docker" BUILD_IN_DOCKER=1 BOARD=mips-malta make -C examples/hello-world/
...

BFD: /data/riotbuild/riotbase/examples/hello-world/bin/mips-malta/hello-world.hex: address 0xffffffff80000000 out of range for Intel Hex file
/opt/mips-mti-elf/2016.05-03/bin/mips-mti-elf-objcopy:/data/riotbuild/riotbase/examples/hello-world/bin/mips-malta/hello-world.hex: Bad value
/data/riotbuild/riotbase/Makefile.include:487: recipe for target '/data/riotbuild/riotbase/examples/hello-world/bin/mips-malta/hello-world.hex' failed
make: *** [/data/riotbuild/riotbase/examples/hello-world/bin/mips-malta/hello-world.hex] Error 1
/home/harter/work/git/RIOT/makefiles/docker.inc.mk:266: recipe for target '..in-docker-container' failed

So the change in Makefile.include fixes the workaround described in 78fdc49

Issues/PRs references

Waiting for #11752 to be accurate with the documentation. But there is not build dependency.

Part of introducing FLASHFILE #8838

[cladmi]

The build indeed failed only because of

Running './dist/tools/pr_check/pr_check.sh master' x
Command output:

	Pull request is waiting for another pull request according to its labels set on GitHub

[kaspar030]

The build indeed failed only because of

we should move those checks to murdock's post-build scripts, so the tags get evaluated at the end of a build.

[cladmi]

The build indeed failed only because of

we should move those checks to murdock's post-build scripts, so the tags get evaluated at the end of a build.

In that case, it was correct as it was waiting for the other PR.

One nice thing would be to say "only static-tests failed" as status in github.

[cladmi]

Rebase now that mips-malta has been dropped.

[MrKevinWeiss]

Seems good now and murdock passes. ACK!

[cladmi]

Thanks for the review.