classes/license: fix manifest to work with deb

classes/license: fix manifest to work with deb

Prepend the license manifest creation call to ROOTFS_POSTPROCESS_COMMAND
instead of appending to ROOTFS_POSTINSTALL_COMMAND. The latter is not
implemented for the deb backend (and probably ought to just be removed
completely), and by using _prepend we can still ensure it occurs before
package info is removed (and before buildhistory in case it is needed
there in future).

(From OE-Core rev: 56fafe083fdb63f156a081344fb1d836e3182bf0)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>