update constructor of FdtWriter

[andreeaflorescu]

There is no need for users to pass an empty array as memory reservations. For the basic use case, the memory reservations are not needed, so users should not be aware of them.

To account for this, we create 2 constructors:

• new -> takes no parameter (basic use case scenario)
• new_with_mem_reserv -> for users that want to pass memory reservations when initializing the FdtWriter.

[andreeaflorescu]

@alexandruag I will merge them. In case @danielverkamp sees something wrong with them, we can provide updates in subsequent PRs.