update_image: -xz_compression is a boolean, so don't require an argument
update_image: -xz_compression is a boolean, so don't require an argument

Rather than using -xz_compression true/false, just use the presence of
the argument to signify xz_compression.

Also, store it as a boolean-readable value, rather than arbitrary
strings.

Signed-off-by: Jeremy Kerr <68a658b44bbaefab3ef5cd231967195fc0ff9d69@ozlabs.org>

open-power/pnor,sannerd/pnor