STM32H7 BSP tree refactoring proposal.

[Karel Gardas]

Hello Sebastian,

I'd like to propose following STM32H7 BSP tree refactoring:

- move BSP's board dependent bits into boards/<manufacturer>/<board 
name> directory

- get rid of very complex (merged for 3 boards already) 
start/system_stm32h7xx.c file

- get rid of start/ext-mem-ctl.c file (as this is split off of 
system_stm32h7xx.c file anyway)

- in board directory use STM32 IDE provided system_stm32h7xx.c board 
specific file whenever supported (e.g. supplied by STM)

- in board directory allow usage of BSP files provided by ST Micro. E.g.

https://github.com/STMicroelectronics/stm32h7b3i-dk
https://github.com/STMicroelectronics/stm32h743i-eval


Pros:
- simplification of merging of future changes in STM provided 
system_stm32h7xx.c files (and others)
- simplification of doing board specific changes

Cons:
- a bit more duplicated code, not in bsp binary build, but in the tree


I already did proposed changes in my branch on github.com: 
https://github.com/karelfv/rtems/tree/stm32h7-boards-refactoring

All three supported BSPs compile with defaults and 7b3i-dk even run on 
my board. Can't test others due to missing hardware.


Please review the directory structure and let me know what do you think 
about it?

No, the tree is not ready for merge yet as this is done in one big 
commit. If you agree with the structure I'll clean it up and do more 
fine grained commits for merge preparation.

Thanks!
Karel