lwIP support

[robamu]

lwIP is supplied as part of some Cube repositories, e.g. for the H7. I think it would be a nice feature if one could use the lwIP contained inside that repository or in a separate cloned repository as link it as STM32::LWIP . The only file lwIP requires to be compiled as a separate static library is the lwipopts.h file. There are some example applications which provide this file, but the first solution might be to expect the user to supply this file.

[robamu]

Implemented and merged in us-irs#18 . However, it requires CMSIS RTOS provided by #189 , which has not been merged here yet. An example would be nice, I only tested that it compiles for various library combinations, but some example files provided by STM are licensed under a different license and I am not sure whether those files can be replaced here. Another solution would be to copy them from the Cube repository during the build process..