makefiles/buildtests.inc.mk: fix 'clean-intermediates' not cleaning

[cladmi]

Contribution description

clean-intermediates should be done per board. Without this it only
tries to clean the default board.

This is a split of #9831 as the original problem mentioned there was different than I thought and unrelated to this fix only.

Testing procedure

Run buildtest in any example without bin at the beginning. Check the bin directory after, only the not intermediates files should be kept:
(it should not be run with variables set from the command line either it triggers another bug!)

rm -rf examples/hello-world/bin && \
  BOARDS="iotlab-m3 samr21-xpro" make --no-print-directory -C examples/hello-world/ buildtest && \
  ls examples/hello-world/bin/*

With this PR, we only have the iotlab-m3 and samr21-xpro outputs

Building for iotlab-m3 ... success.
Building for samr21-xpro ... success.
examples/hello-world/bin/iotlab-m3:
hello-world.elf  hello-world.hex  hello-world.map

examples/hello-world/bin/samr21-xpro:
hello-world.bin  hello-world.elf  hello-world.map

With master we have

Building for iotlab-m3 ... success.
Building for samr21-xpro ... success.
examples/hello-world/bin/iotlab-m3:
application_hello-world    auto_init.a  boards_common_iotlab    core.a            cortexm_common_periph    cpu.a      hello-world.elf  isrpipe                  newlib_syscalls_default.a  periph_common    pm_layered.a  stdio_uart.a    stm32_common_periph    sys.a
application_hello-world.a  board        boards_common_iotlab.a  cortexm_common    cortexm_common_periph.a  drivers    hello-world.hex  isrpipe.a                periph                     periph_common.a  riotbuild     stm32_common    stm32_common_periph.a  tsrb
auto_init                  board.a      core                    cortexm_common.a  cpu                      drivers.a  hello-world.map  newlib_syscalls_default  periph.a                   pm_layered       stdio_uart    stm32_common.a  sys                    tsrb.a

examples/hello-world/bin/native:

examples/hello-world/bin/samr21-xpro:
application_hello-world    auto_init.a  core            cortexm_common.a         cpu      drivers.a        hello-world.map  newlib_syscalls_default    periph.a         pm_layered    sam0_common         sam0_common_periph.a  sys    tsrb.a
application_hello-world.a  board        core.a          cortexm_common_periph    cpu.a    hello-world.bin  isrpipe          newlib_syscalls_default.a  periph_common    pm_layered.a  sam0_common.a       stdio_uart            sys.a
auto_init                  board.a      cortexm_common  cortexm_common_periph.a  drivers  hello-world.elf  isrpipe.a        periph                     periph_common.a  riotbuild     sam0_common_periph  stdio_uart.a          tsrb

Issues/PRs references

Split out of #9831 and found while working on #9742

[cladmi]

Sorry to ask you to review again here, I extracted the most urgent part out of #9831

[jcarrano]

I could reproduce the issue and the fix fixes it.

[jia200x]

Great, thank you!

[cladmi]

Thank you for reviewingy