We recently introduced a pySim.utils.build_construct() wrapper around
the raw call of the construct.build() method. So far, this wrapper
was only used from pySim.tlv, but let's also use it from
pySim.filesystem.
Basically, whenever we use parse_construct(), we should use
build_construct() as the inverse operation.
Change-Id: Ibfd61cd87edc72882aa66d6ff17861a3e918affb