MPU_ALLOW_FLASH_WRITE is defined only for ARM platforms.

Remove MPU_ALLOW_FLASH_WRITE from prj.conf and put it in
boot/zephyr/Kconfig enabling it only for every ARM_MPU
equipped device.

Signed-off-by: Rajavardhan Gundi <rajavardhan.gundi@intel.com>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 49627e0..eafdb94 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -11,6 +11,7 @@
 config MCUBOOT
 	default y
 	bool
+	select MPU_ALLOW_FLASH_WRITE if ARM_MPU
 
 config BOOT_USE_MBEDTLS
 	bool