[Silicon Labs] Rename targets

Due to limitation in the mbed website backend (board names need to be <= 19 characters), we are shortening the CLI target names from THUNDERBOARD to TB.
@screamerbg

targets/TARGET_Silicon_Labs/TARGET_SL_RAIL/mbed_lib.json

@@ -21,10 +21,10 @@
         "pti-dframe-pin": 13
     },
     "target_overrides": {
-        "THUNDERBOARD_SENSE": {
+        "TB_SENSE_1": {
             "sl-rail.has-2p4": true
         },
-        "THUNDERBOARD_SENSE_12": {
+        "TB_SENSE_12": {
             "sl-rail.has-2p4": true
         },
         "EFR32MG1_BRD4150": {

targets/targets.json

@@ -2299,7 +2299,7 @@
         },
         "public": false
     },
-    "THUNDERBOARD_SENSE": {
+    "TB_SENSE_1": {
         "inherits": ["EFR32MG1P233F256GM48"],
         "device_has": ["ANALOGIN", "ERROR_PATTERN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LOWPOWERTIMER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES"],
         "forced_reset_timeout": 5,
@@ -2396,7 +2396,7 @@
         "release_versions": ["2", "5"],
         "public": false
     },
-	"THUNDERBOARD_SENSE_12": {
+	"TB_SENSE_12": {
         "inherits": ["EFR32MG12P332F1024GL125"],
         "device_has": ["ANALOGIN", "ERROR_PATTERN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LOWPOWERTIMER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES"],
         "forced_reset_timeout": 5,