build: images: squashfs: add help, fix description

build: images: squashfs: add help, fix description

add help text for `TARGET_SQUASHFS_BLOCK_SIZE` to match the only valid
settings accepted by `mksquashfs4` ("block size not power of two or not
between 4096 and 1Mbyte") thus for this setting in "KB", the set:
  `4, 8, 16, 32, 64, 128, 256, 512, 1024`

replace `squashfs-lzma` with `squashfs` in the description for
`TARGET_ROOTFS_SQUASHFS` because it has various compressions, and not
just lzma as it did in the past

cosmetic change with no functional effect

Signed-off-by: Tony Butler <spudz76@gmail.com>