Add support for specifying data bits, stop bits and parity at
runtime.

Introduce feature periph_uart_modecfg for uart_mode() till all
other CPUs implement it.

STM32 L1, F1, F2, F4 supports following modes:

* 7E1, 7E2
* 7O1, 7O2
* 8N1, 8N2
* 8E1, 8E2
* 8O1, 8O2

STM32 L0, L4, F0, F3, F7 supports following modes:

* 6E1, 6E2
* 6O1, 6O2
* 7E1, 7E2
* 7O1, 7O2
* 7N1, 7N2
* 8N1, 8N2
* 8E1, 8E2
* 8O1, 8O2

Use USART_CR1_M1 macro to detect 7-bit support because
even inside one family there could be devices that don't
support 7-bit mode. So just using a family macro is not
enough.

As stated in the datasheets for L0, L4, F0, F3, F7 devices,
data bits can only be changed when UART is disabled (UE=0).
Introduce uart_stop() routine to satisfy this requirement.

STM32 UART adds parity to the MSB of a byte to send. The same
also applies to the received bytes. As a result this bit must
be masked in order to get the pure data.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>