Disable MPU RT1050 due to memory map

Disable the MPU on the RT1050 since this target has a memory map that
is incompatible with the default MPU driver.

targets/targets.json

@@ -1845,7 +1845,8 @@
             "XIP_BOOT_HEADER_DCD_ENABLE=1",
             "SKIP_SYSCLK_INIT",
             "FSL_FEATURE_PHYKSZ8081_USE_RMII50M_MODE",
-            "SDRAM_IS_SHAREABLE"
+            "SDRAM_IS_SHAREABLE",
+            "MBED_MPU_CUSTOM"
         ],
         "inherits": ["Target"],
         "detect_code": ["0227"],
@@ -1867,8 +1868,7 @@
             "SERIAL",
             "SPI",
             "SPISLAVE",
-            "STDIO_MESSAGES",
-            "MPU"
+            "STDIO_MESSAGES"
         ],
         "release_versions": ["2", "5"],
         "features": ["LWIP"],