Set OUTPUT_EXT for ARM_MUSCA_S1 and ARM_MUSCA_B1

By default, Mbed CLI 2 + CMake builds both .bin and .hex images,
unless a target specifies OUTPUT_EXT. The post binary hook for
ARM_MUSCA_S1 and ARM_MUSCA_B1 is run on .bin images only, so we need
to prevent .hex images from being generated and confusing users.

Author: LDong-Arm

targets/targets.json

@@ -4855,7 +4855,8 @@
             "ARMCLANG",
             "GNUARM"
         ],
-        "tfm_delivery_dir": "TARGET_ARM_SSG/TARGET_MUSCA_B1"
+        "tfm_delivery_dir": "TARGET_ARM_SSG/TARGET_MUSCA_B1",
+        "OUTPUT_EXT": "bin"
     },
     "ARM_MUSCA_S1": {
         "inherits": [
@@ -4910,7 +4911,8 @@
         "tfm_delivery_dir": "TARGET_ARM_SSG/TARGET_MUSCA_S1",
         "detect_code": [
             "5009"
-        ]
+        ],
+        "OUTPUT_EXT": "bin"
     },
     "RZ_A1XX": {
         "inherits": [