hw/drivers: Lack of simple UART string transmit API

Currently, there is no easy way to send a full string over a UART interface.
You have to send each character manually, like this:
```
char *str = "Hello World!";
char *ptr = str;

while (*ptr) {
hal_uart_blocking_tx(MY_UART, *ptr++);
}
hal_uart_blocking_tx(MY_UART, '\n');
```
For the console, there is `console_printf()`, but there isnt similar function for general UARTs.

**Proposal:**
Add function, for example:
`void hal_uart_transmit(int uart, char *str);`
that would send the whole string over the selected UART.

**Example usage:**
`hal_uart_transmit(1, "Hello World!\n");`

Start by reading the UART HAL around hal_uart_blocking_tx and compare it with the existing console_printf API. Define the transmit API's expected string and blocking behavior, then verify that a selected UART can send a complete string including the example newline.

Written by the indexing model from the issue text.