boards: arm: add support for NUCLEO-L011K4 board

Support for the NUCLEO-L011K4 development board with STM32L011K4 SoC.

Although the SoC only contains 16K flash and 2K RAM, it has been tested
on a number of samples and has worked as expected.

Signed-off-by: Steven Daglish <s.c.daglish@gmail.com>