[archiso] Rename root-image to airootfs

Two purposes:
 * systemd filename friendly: "-" is used for "/" and must be escaped with "\x20"
 * ISO9660 filename friendly: "-" is replaced with "_"
[archiso] mkarchiso: Only use ext4 as filesystem for airootfs
[archiso] Always use a 32GiB filesystem size for airootfs

There is no big overhead.
Space usage in ext4-1G is ~0.4M while in ext4-32G is ~4M.
Just use a big value to make all people happy.
[archiso] mkarchiso: Add -c switch to specify squashfs compression type.
[archiso] Drop aitab support

Always create one filesystem of a fixed size (32G), format (ext4) and
know name "airootfs".

Simplify logic a lot.
[archiso] rename airootfs.fs.sfs to airootfs.sfs

* Make it ISO9660 friendly (extra dot "." is replaced by "_")
* Was used when support both .fs.sfs and .sfs
[archiso] Merge _mkfs() and _mksfs() in _mkairootfs()
[archiso] Rework _mount_fs() to _mount_airootfs() same for umount

Also remove _show_space_usage() irrelevant now when fs size is fixed and big
[archiso] Remove _is_directory_changed()

This is a task done in some way by run_once() on build.sh
[archiso] Minor change
[archiso] command_install() remove "already executed" logic

This is done better with run_once() in build.sh
[archiso] _chroot_init() remove "already runned" logic

This is done better with run_once() in build.sh
[archiso] Rework checksum function