Add STM8L151K4 board support for RM0031 STM8L Medium Density Devices

[TG9541]

STM8L151K4 stands for "Medium density" devices as described in RM0031.

The STM8L-Discovery provides support for the devices mentioned above but it also provides LCD support by default. Issue #268 discussed support for STM8L151 devices that meet the criteria above (i.e. not Low Density) but also without LCD peripherals, and @Eelkhoorn sent me an STM8L151K4 sample (thanks again :-) ). That's more than enough reason to provide a target that's easy to identify and ready to use.

[TG9541]

Pin	GPIO	Function
32	PA0	SWIM
1	PA1	NRST
2	PA2	[USART1_TX]
3	PA3	[USART1_RX]
27	PC2	USART1_RX
28	PC3	USART1_TX

[TG9541]

This handy overview of the main STM8L151K4 GPIO was made by @Eelkhoorn:

Thanks for the permission to use functions here!

[TG9541]