Update the watchdog timing requirements:
1. The watchdog should trigger at, or after the timeout value.
2. The watchdog should trigger before twice the timeout value.
* capitalize driver class name,
* reword test docs for the driver & HAL,
* capitalize Mbed name,
* reword the comments explaining the SERIAL_FLUSH_TIME_MS macro.
The latest rebase of the watchdog feature branch introduced errors in compiling
watchdog tests due to missing headers.
- Watchdog HAL API test
Include missing header files to main.cpp (mbed_wait_api.h, stdlib.h).
- Watchdog HAL API timing test
Include missing header files to main.cpp (us_ticker_api.h).
- Watchdog Driver API test
Include missing header files to main.cpp (mbed_wait_api.h, stdlib.h).
Inject mbed namespace into main.cpp to fix Watchdog name resolution.