image.bbclass: support duplicate compression types
When a derived distro adds a certain type, say zip, to
COMPRESSIONTYPES and later OE-core does the same, we end up with the
type being listed twice, and that would have undesired effects
(commands generated twice).
So to support such loosely coupled extension, we de-duplicated the
list of types first.
Alternatively, such a situation could also be treated as error. But that
seems unnecessary because typically commands for the same type will also
do the same thing.
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com> Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
