Modified call-before-main mechanism

Added a new function (mbed_sdk_init) that can be used by the SDK itself to
do initializations before main() is called.

Author: Bogdan Marinescu

libraries/mbed/common/retarget.cpp

@@ -398,22 +398,31 @@ extern "C" WEAK void __cxa_pure_virtual(void) {
 
 // ****************************************************************************
 // mbed_main is a function that is called before main()
+// mbed_sdk_init() is also a function that is called before main(), but unlike
+// mbed_main(), it is not meant for user code, but for the SDK itself to perform
+// initializations before main() is called.
 
 extern "C" WEAK void mbed_main(void);
 extern "C" WEAK void mbed_main(void) {
 }
 
+extern "C" WEAK void mbed_sdk_init(void);
+extern "C" WEAK void mbed_sdk_init(void) {
+}
+
 #if defined(TOOLCHAIN_ARM)
 extern "C" int $Super$$main(void);
 
 extern "C" int $Sub$$main(void) {
+    mbed_sdk_init();
     mbed_main();
     return $Super$$main();
 }
 #elif defined(TOOLCHAIN_GCC)
 extern "C" int __real_main(void);
 
 extern "C" int __wrap_main(void) {
+    mbed_sdk_init();
     mbed_main();
     return __real_main();
 }
@@ -424,6 +433,7 @@ extern "C" int __wrap_main(void) {
 // code will call a function to setup argc and argv (__iar_argc_argv) if it is defined.
 // Since mbed doesn't use argc/argv, we use this function to call our mbed_main.
 extern "C" void __iar_argc_argv() {
+    mbed_sdk_init();
     mbed_main();
 }
 #endif

libraries/mbed/targets/hal/TARGET_NXP/TARGET_LPC176X/TARGET_UBLOX_C027/platform_init.c

@@ -3,7 +3,7 @@
 #include "wait_api.h"
 
  // called before main
-void mbed_main()
+void mbed_sdk_init()
 {
     gpio_t modemEn, modemRst, modemPwrOn, modemLvlOe, modemILvlOe, modemUsbDet;
     gpio_t gpsEn, gpsRst, led, modemRts;