travis: Build NUCLEO_L073RZ instead of B_L072Z_LRWAN1 for stm32 job.

The NUCLEO_L073RZ build is slightly bigger and this MCU has only 192k flash
so this helps to catch flash overflow.

.travis.yml

@@ -56,7 +56,7 @@ jobs:
         - make ${MAKEOPTS} -C ports/stm32 BOARD=PYBD_SF2
         - make ${MAKEOPTS} -C ports/stm32 BOARD=PYBD_SF6 NANBOX=1 MICROPY_BLUETOOTH_NIMBLE=0 MICROPY_BLUETOOTH_BTSTACK=1
         - make ${MAKEOPTS} -C ports/stm32 BOARD=NUCLEO_H743ZI CFLAGS_EXTRA='-DMICROPY_PY_THREAD=1'
-        - make ${MAKEOPTS} -C ports/stm32 BOARD=B_L072Z_LRWAN1
+        - make ${MAKEOPTS} -C ports/stm32 BOARD=NUCLEO_L073RZ
         - make ${MAKEOPTS} -C ports/stm32 BOARD=STM32L476DISC
         - make ${MAKEOPTS} -C ports/stm32 BOARD=NUCLEO_WB55
         - make ${MAKEOPTS} -C ports/stm32/mboot BOARD=PYBD_SF6