ulab not compatible with enabling '_thread' module from the make command :
- building ulab with thread enabled in MicroPython :
`$ make -j8 CFLAGS_EXTRA='-DMICROPY_PY_THREAD=1' BOARD=PYBV11 USER_C_MODULES=../../../ulab all`
yields a firmware with '_thread' module but without 'ulab' module, because 'CFLAGS_EXTRA' is used in the make command and in 'ulab/code/micropython.mk', so the value of the last is ignored;
- the solution is to use 'override' and '+=' in in last line of 'ulab/code/micropython.mk' :
`override CFLAGS_EXTRA += -DMODULE_ULAB_ENABLED=1`
