docs: update link to stretch image
docs: update link to stretch image

Give link to the newer image (it's gzip'ed and has password-less ssh).
Also we now put all kernel command line arguments into kernel config
(CONFIG_CMDLINE), so passing them explicitly to qemu is not necessary.

xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller