mkimage/mkknlimg: fix for XZ-compressed kernels

unxz complains about garbage data, so tell it to stop after the first
stream has been correctly decompressed.

Also, the kernel image may contain the XZ marker twice, and we
are only interested in the second one.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>