samples: mpu_test: Add Allow Flash Write config

This patch adds a configuration file that tests Flash writes with MPU
enabled.

[david.brown: Put options in prj.conf with comments as per review
 feedback]

Signed-off-by: Vincenzo Frascino <[email protected]>
Signed-off-by: David Brown <[email protected]>

Author: Vincenzo Frascino

samples/mpu/mpu_test/README.rst

@@ -46,6 +46,10 @@ single thread: :file:`prj_single.conf`:
 
    $ make BOARD=v2m_beetle CONF_FILE=prj_single.conf run
 
+To build a version that allows writes to the flash device, edit
+``prj.conf``, and follow the directions in the comments to enable the
+proper configs.
+
 Sample Output
 =============
 

samples/mpu/mpu_test/prj.conf

@@ -1 +1,14 @@
 CONFIG_CONSOLE_SHELL=y
+
+### In order to allow the flash device to be written to, this line will
+### need to be uncommented.  Note that on some devices (MCUX/NXP), this
+### needs to be enabled to even enable CONFIG_FLASH, as the probe needs
+### to write to flash.  Also, on NXP, enabling flash write also requires
+### that execute from RAM be enabled, as this is needed to operate the
+### flash device.
+# CONFIG_MPU_ALLOW_FLASH_WRITE=y
+# CONFIG_FLASH=y
+
+### These can be uncommented to enable the flash device on the
+### STM32F4X devices.
+# CONFIG_SOC_FLASH_STM32=y