[microTVM] Create per-board default and project option for TVM heap size

[areusch]

Currently the heap size in the Zephyr host_driven microTVM is fixed, but each board has a variable amount of memory and it should be tailored at least per-board. It would also be nice to have a ProjectOption to override this setting in case the user wants to generate a project with smaller footprint.

@mkatanbaf will take a look

cc @alanmacd @gromero @mehrdadh