travis: Change an stmhal rule to build PYBV11 instead of default PYBV10.

This allows to test the PYBV11 target as well as the network drivers
without adding another rule.  It also removes the need to use -B,
side-stepping the issue of whether or not -B works with qstr auto
generation.

.travis.yml

@@ -39,7 +39,7 @@ script:
   - make -C bare-arm
   - make -C qemu-arm test
   - make -C stmhal
-  - make -C stmhal -B MICROPY_PY_WIZNET5K=1 MICROPY_PY_CC3K=1
+  - make -C stmhal BOARD=PYBV11 MICROPY_PY_WIZNET5K=1 MICROPY_PY_CC3K=1
   - make -C stmhal BOARD=STM32F7DISC
   - make -C stmhal BOARD=STM32L476DISC
   - make -C teensy