Additional make build arguments

[zdebanos]

With @ppisa, we propose a new option to be specified in the model's dialog: the additional make build arguments. With this, the user can specify arguments that are specific to the model. The reason we bring this up is the new Nuttx timerhook I am working on now.

However, with my update, the repository would contain 3 makefike templates (the normal clock_nanosleep one, the systemtick one and the new one). Also a new Nuttx main.c would have to be introduced. With additional build arguments, one makefile and main.c would suffice. My timerhook would then be chosen by the additional argument in pysimCoder. The preprocessor magic would then do all the work.