[Bug] make ev3dev-armel does not use correct mpy-cross

[laurensvalk]

Describe the bug

mpy-cross not found at /src/micropython/mpy-cross/build/mpy-cross, please build it first
../../micropython/py/mkrules.mk:188: recipe for target 'build-armel/frozen_content.c' failed
make: *** [build-armel/frozen_content.c] Error 1
make: *** Waiting for unfinished jobs....
make: *** [Makefile:56: ev3dev-armel] Error 2

It seems like it should be using mpy-cross/build-armel, which does get built. If I rename that folder to mpy-cross/build, then make ev3dev-armel proceeds.

To reproduce
Steps to reproduce the behavior:

1. Clean everything
2. make ev3dev-armel

[dlech]

Did you rebuild the pybricks-ev3dev_armel docker container? The setup script changed to add the required environment variable.