fix help message for mkimage debootstrap with components

debootstrap needs the suite as the second argument, for this the script
reorders arguments beginning with a minus but components separated by
space, as stated by the help message, is not handled and will lead to
the rootfs being passed as suite to debootstrap.
The poor mans solution is to fix the help message to pass the long
option as one argument.

Signed-off-by: Julian Taylor <jtaylor.debian@googlemail.com>
Merge pull request #8346 from juliantaylor/mkimage-debootstrap

fix help message for mkimage debootstrap with components