arm-bsp/classes: Use :append to add to IMAGE_TYPES in wic_nopt

IMAGE_TYPES += "wic.nopt" is effective if the bbclass is included
using IMAGE_CLASSES, but not if included directly (using inherit) due to
file parse ordering.

To support applying wic_nopt locally (i.e. for certain image recipes but
not others), change to use :append.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/classes/wic_nopt.bbclass

@@ -1,7 +1,7 @@
 # This class removes the empty partition table header
 # in the WIC file when --no-table WKS option is used
 
-IMAGE_TYPES += "wic.nopt"
+IMAGE_TYPES:append = " wic.nopt"
 
 CONVERSIONTYPES += "nopt"