New driver interface register/save/restore

This MR implements the new driver interface and checkpoint allocation as already discussed offline.

I will have to run again some tests to verify that I didn't introduce any bugs while refactoring for the merge request, but in general this should work.

I open this MR so you can already have a look.

src/cmake/Sytare.cmake

@@ -33,6 +33,7 @@ enable_language(C)
 ### Compiler flags
 
 add_compile_options(
+    -std=c99
 	-mmcu=${MCU}        # same MCU for all code that is compiled
 	-g                  # always generate symbols for debugging (doesn't hurt)
 	-fno-common         # avoid COMMON symbols so we can rename sections