musca: Add detect_code for MUSCA B1

Wtihout the detect_code, `mbed-tools detect` will fail to detect any
connected ARM_MUSCA_B1 boards with the error:

    ERROR: Could not identify a board with the product code: '5007'.
    No connected Mbed devices found.

Add the detect_code for MUSCA B1 to targets.json so the board can be
reliably detected.

Fixes ARMmbed/mbed-tools#272

Author: Patater

targets/targets.json

@@ -4882,6 +4882,9 @@
             "GNUARM"
         ],
         "tfm_delivery_dir": "TARGET_ARM_SSG/TARGET_MUSCA_B1",
+        "detect_code": [
+            "5007"
+        ],
         "OUTPUT_EXT": "bin"
     },
     "ARM_MUSCA_S1": {