1. With mbed OS 5, mbed_sdk_init() is ensured to call before C++ global object constructor.
2. Refine startup file with GCC_ARM toolchain related to this modification.
Serial implementation uses different vector handlers for sync/async calls respectively. The issue can be reproduced with the following flow:
1. Register sync mode callback with Serial.attach().
2. Sync call with Serial.putc()/getc().
3. Change to async call with Serial.write()/read().
4. Change back to sync call with Serial.putc()/getc().
Now, vector handller is still for async mode, not for sync mode.
To fix it:
1. Introduce internal function serial_enable_interrupt() for both sync/async vector handler enable/disable.
Original HAL function serial_irq_set() is reduced to call it for sync mode vector handler enable/disable.
2. Introduce internal function serial_rollback_interrupt() to roll back sync mode vector handler at end of async transfer.
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ccli8