Make the EXTI-TIM4 serial interface compatible with STM8L

[TG9541]

A proof-of-concept is in the repo STM8 eForth STM8L051LED.

Here are some requirements for the solution:

1. STM8 eForth should be easier to configure and apply Note: aggregate EXTI-TIM4 code to a few central files that can be takren into board folders if necessary
2. all GPIOs capable of EXTI shall be supported Note: this means using the bit patterned EXTI0..EXTI7
3. interrupt vectors should be easier to manage Note: remove dummy code from boardcore.inc
4. the simulated serial should at least work in full-duplex mode (sser-fdx) with STM8L051F3 Note: the code should be modular and sound but testing specific configurations can be done in applications
5. it should be possible to add half-duplex (sser_hdx) and other STM8L devices later on Note: this optional

[TG9541]

At least on the STM8L051F3 sser_hdx also works. The optional "point 5." will thus be met.

[TG9541]

While I was at it I made TIM4_ARR and TIM4_PSCR configuration items and tested baud rates higher than 9600 baud using the simulated serial interface. This is what worked:

Rate	CTIM4ARR	CTIM4PSCR	sser-fdx	sser-hdx
600	7	0xCF	yes*	yes*
1200	6	0xCF	yes*	yes*
2400	5	0xCF	yes*	yes*
4800	4	0xCF	yes	yes
9600	3	0xCF	yes	yes
19200	2	0xCF	yes	yes
38400	1	0xCF	yes	yes
57600	1	0x89	(yes)	yes
115200	0	0x89	no	(yes)

*) untested but very likely to work - due to a 4 bit pre-scaler index STM8L devices can also use 300, 150, 75 downto 2.34375 baud - maybe that's useful for communication with a flashlight ;-)

Full-duplex at 115200 baud was too much for the combined TIM4 Rx/Tx state machine, and it stopped working. 115200 with half-duplex worked fine (using the new "-r" option of codeload.py, see #320) but I didn't test it with e4thcom since the baud rate in version 0.8.0.64 appears to be limited to 57600.

To be safe I would suggest limiting the usage of the simulated serial in full-duplex to 38400 baud and in half-duplex to 57600 baud.