ELFFile: allow filtering of sections by type in iter_sections (#345)

ELFFile: allow filtering of sections by type in iter_sections (#345)

As stated in the corresponding issue, we can already filter
the output of Dynamic.iter_tags() by the type of the tag
we're looking for.

Let's adapt the iteration over the sections of the ELF file
so that it only yields sections of a certain type if the
optional type parameter is passed to iter_sections().

By doing this we can also simplify two call sites inside
the ELFFile class.

Fixes: #344