Merge pull request ARMmbed#89 from linlingao/disable_mpu

Disable MPU for now

Author: linlingao

targets/targets.json

@@ -7613,8 +7613,9 @@
     },
     "CC3220SF": {
         "inherits": ["CC32XX"],
+        "macros": ["MBED_MPU_CUSTOM"],
         "components": ["SD"],
-        "device_has": ["USTICKER", "SERIAL", "SERIAL_FC", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "INTERRUPTIN", "SPI", "ANALOGIN", "FLASH", "TRNG", "MPU"],
+        "device_has": ["USTICKER", "SERIAL", "SERIAL_FC", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "INTERRUPTIN", "SPI", "ANALOGIN", "FLASH", "TRNG"],
         "core": "Cortex-M4",
         "bootloader_supported": true,
         "device_name": "CC3220SF"