Add support for STM8L devices

[TG9541]

Add support for STM8L051F3 devices:

• add include, and e4thcom EFR files
• provide core functionality (serial console, NVM, background task)

[TG9541]

I'm closing this issue for now:

• the basic stuff works (console with USART1, NVM, backgound task)
• simulated UART configurations are untested (the workings of the interrupts is different)
• there is an e4thcom EFR file: STM8L051.efr

To use this variant do the following:

• connect pin1 to RxD
• connect pin2 to TxD
• connect pin3 to SWIM
• connect pin4 to NRST
• connect pin7 to GND
• connect pin8 to 3.3V
• run make BOARD=STM8L051F3 flash
• run a terminal at 9600n81 (e.g. use e4thcom)

What's missing is some docs, more configurations, and a lot of testing.

[TG9541]

In the meantime support and testing for STM8L has improved:

• Issues have been fixed and support has improved:
  • STM8L: wrong peripherals addresses #202 bug-fix
  • STM8L-DISCOVERY #267 Medium Density support
  • Use generic chip type identifiers #307 generic chip IDs
  • Make the EXTI-TIM4 serial interface compatible with STM8L #319 simulated serial interface HDX
  • Stm8l fdx #321 simulated serial interface FDX
  • Update STM8L.efr #325 generic STM8L.efr
  • fixes #330 Move EEPROMBASE from forth.asm to target.inc #333 fix EEPROMBASE
  • STM8L-DISCOVERY: enable UART output #335 bug-fix
• Peripherals support is available at Eelkhoorn/stm8-peripherals-forth
• An example for modular-build support can be found in TG9541/stm8l051led