Extract file system's infos from image_head and move them in file_system_info

This commit impacts a lot of code, but it mostly consist of replacing one
type by another. Some functions are renamed to make them more logical given
the change.

This is an intermediate step. Some functions must access image_hdr and
fs_info because the values are not totally separated, but it will be fixed
in a later commit. For now the code is still purely version 0001.

Signed-off-by: Patryck Rouleau <pfrouleau@gmail.com>