Introduce `subtarget_sdk_init` startup hook.

The `mbed_sdk_init` startup hook is implemented at the NRF52-series level and so is unavailable for override. This commit adds an additional startup hook for NRF52 subtargets to perform any other startup initialization required.

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/reloc_vector_table.c

@@ -57,6 +57,9 @@
     uint32_t nrf_dispatch_vector[NVIC_NUM_VECTORS] @ ".nvictable";
 #endif
 
+#include "platform/mbed_toolchain.h"
+#include "subtarget_init.h"
+
 extern uint32_t __Vectors[];
 
 #define VECTORS_FLASH_START __Vectors
@@ -113,7 +116,6 @@ void nrf_reloc_vector_table(void)
 #endif
 }
 
-
 void mbed_sdk_init(void)
 {
 	if (STDIO_UART_RTS != NC) {
@@ -122,4 +124,6 @@ void mbed_sdk_init(void)
 		/* Set STDIO_UART_RTS as gpio driven low */
 		gpio_write(&rts, 0);
 	}
+
+        subtarget_sdk_init();
 }

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/subtarget_init.c

@@ -0,0 +1,24 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2021 ARM Limited
+ * Copyright (c) 2021 Embedded Planet, Inc.
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "subtarget_init.h"
+#include "platform/mbed_toolchain.h"
+
+MBED_WEAK void subtarget_sdk_init(void) {
+    /* Do nothing by default */
+}

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/subtarget_init.h

@@ -0,0 +1,39 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2021 ARM Limited
+ * Copyright (c) 2021 Embedded Planet, Inc.
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef _NORDIC_SUBTARGET_INIT_
+#define _NORDIC_SUBTARGET_INIT_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Since Mbed's `mbed_sdk_init` hook is used by the NRF52 family code, this
+ * initialization hook is provided so subtargets may implement their own startup
+ * initialization code, if necessary.
+ * 
+ * By default, it is a blank function that is declared a "weak" symbol
+ */
+void subtarget_sdk_init(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _NORDIC_SUBTARGET_INIT_ */