For example, currently, the IMAGE_FSTYPES are:

    jffs2 jffs2.sum \
    cramfs \
    ext2 ext2.gz ext2.bz2 ext2.lzma \
    ext3 ext3.gz \
    ext4 ext4.gz \
    btrfs \
    iso \
    hddimg \
    squashfs squashfs-xz squashfs-lzo \
    ubi ubifs multiubi \
    tar tar.gz tar.bz2 tar.xz tar.lz4 \
    cpio cpio.gz cpio.xz cpio.lzma cpio.lz4 \
    vmdk \
    vdi \
    qcow2 \
    hdddirect \
    elf \
    wic wic.gz wic.bz2 wic.lzma \
"

ext2, ext2.gz ext2.bz2 ext2.lzma can be divded into two part:
image type: ext2
compress format: gz, bz2, lzma

The similar to others, this can make things easier. In another world, make IMAGE_FSTYPES not contain compress format, and then once the user set the compress type, it can be compressed into any types.