Without free-up of peripheral pins, peripheral pins of the same peripheral may
share by multiple ports after port iteration, and this peripheral may fail with
pin interference.
Better IP initialization sequence:
1. Configure IP pins
2. Select IP clock source and then enable it
3. Reset the IP (SYS_ResetModule)
NOTE1: IP reset takes effect regardless of IP clock. So it doesn't matter if
IP clock enable is before IP reset.
NOTE2: Non-configured pins may disturb IP's state, so IP pinout first and then
IP reset.
NOTE3: IP reset at the end of IP initialization sequence can cover unexpected
situation.
With support for checking H/W UART initialized or not, we can simplify stdio management:
1. When serial_init(&stdio_uart) calls in, just set the 'stdio_uart_inited' flag.
2. When serial_free(&stdio_uart) calls in, just clear the 'stdio_uart_inited' flag.
Except above, we needn't make special handling with 'stdio_uart'.
The same H/W UART may be shared by multiple serial_t objects. This fix tries to avoid
re-configuring the same H/W UART in serial_init() when there are multiple serial_t
objects constructed. To re-configure UART, call serial_baud() and serial_format()
explicitly. This can avoid confusion when e.g. a newly constructed serial_t object
changes baudrate unexpectedly in serial_init().
Chun-Chieh Li
Russ Butler