tools: bitfield_gen: use itertools.chain instead of concatenating lists
tools: bitfield_gen: use itertools.chain instead of concatenating lists

This isn't efficient in Python, and moreover in python3 these methods
return iterables, not lists.

cmr/seL4,zhicheng/seL4,zhicheng/seL4,cmr/seL4,zhicheng/seL4,cmr/seL4