Add a platform config to disable the MPU

Add the platform config DISABLE_MPU to allow the MPU to be disabled
from an application.

hal/mpu/mbed_mpu_v7m.c

@@ -20,7 +20,7 @@
 
 #if ((__ARM_ARCH_7M__ == 1U) || (__ARM_ARCH_7EM__ == 1U) || (__ARM_ARCH_6M__ == 1U)) && \
     defined (__MPU_PRESENT) && (__MPU_PRESENT == 1U) && \
-    !defined(MBED_MPU_CUSTOM)
+    !defined(MBED_MPU_CUSTOM) && !MBED_CONF_PLATFORM_DISABLE_MPU
 
 #if !DEVICE_MPU
 #error "Device has v7m MPU but it is not enabled. Add 'MPU' to device_has in targets.json"

hal/mpu/mbed_mpu_v8m.c

@@ -20,7 +20,7 @@
 
 #if ((__ARM_ARCH_8M_BASE__ == 1U) || (__ARM_ARCH_8M_MAIN__ == 1U)) && \
     defined (__MPU_PRESENT) && (__MPU_PRESENT == 1U) && \
-    !defined(MBED_MPU_CUSTOM)
+    !defined(MBED_MPU_CUSTOM) && !MBED_CONF_PLATFORM_DISABLE_MPU
 
 #if !DEVICE_MPU
 #error "Device has v8m MPU but it is not enabled. Add 'MPU' to device_has in targets.json"

hal/mpu_api.h

@@ -26,7 +26,7 @@
 extern "C" {
 #endif
 
-#if DEVICE_MPU
+#if DEVICE_MPU && !MBED_CONF_PLATFORM_DISABLE_MPU
 
 /**
  * \defgroup hal_mpu MPU hal

platform/mbed_lib.json

@@ -128,6 +128,10 @@
         "fatal-error-auto-reboot-enabled": {
             "help": "Setting this to true enables auto-reboot on a fatal error.",
             "value": false
+        },
+        "disable-mpu": {
+            "help": "Disable the MPU to save rom.",
+            "value": false
         }
     },
     "target_overrides": {

platform/mbed_mpu_mgmt.c

@@ -21,7 +21,7 @@
 #include "hal/mpu_api.h"
 #include <limits.h>
 
-#if DEVICE_MPU
+#if DEVICE_MPU && !MBED_CONF_PLATFORM_DISABLE_MPU
 
 static uint16_t mem_xn_lock;
 static uint16_t mem_wn_lock;

platform/mbed_mpu_mgmt.h

@@ -32,7 +32,7 @@
 extern "C" {
 #endif
 
-#if DEVICE_MPU
+#if DEVICE_MPU && !MBED_CONF_PLATFORM_DISABLE_MPU
 
 /** Lock ram execute never mode off
  *