travis: Build STM32 F7 and L4 boards under Travis CI.

The STM32 F7 and L4 boards use significantly different code to the F4
boards so it's important to test them with CI.  To keep CI build times
within a reasonable limit the STM32F4DISC board is no longer built, it's
anyway very similar to the standard F4 build for PYBv1.0.

.travis.yml

@@ -40,7 +40,8 @@ script:
   - make -C qemu-arm test
   - make -C stmhal
   - make -C stmhal -B MICROPY_PY_WIZNET5K=1 MICROPY_PY_CC3K=1
-  - make -C stmhal BOARD=STM32F4DISC
+  - make -C stmhal BOARD=STM32F7DISC
+  - make -C stmhal BOARD=STM32L476DISC
   - make -C teensy
   - make -C cc3200 BTARGET=application BTYPE=release
   - make -C cc3200 BTARGET=bootloader  BTYPE=release