image.bbclass: image_types.bbclass is a must

image.bbclass: image_types.bbclass is a must

The image_types.bbclass is a must since we use this in image.bbclass:

[snip]
python () {
    deps = " " + imagetypes_getdepends(d)
    d.appendVarFlag('do_rootfs', 'depends', deps)
[snip]

The imagetypes_getdepends() is defined in image_types.bbclass. Use "+="
to replace "?=" since it is a must, so that the user can use
"IMAGE_CLASSES = foo.bbclass" in local.conf to add their own image
class.

NOTE: the IMAGE_CLASSES_append = " foo" doesn't work since we use this
in image.bbclass:
 IMAGE_CLASSES += "image_types"
 inherit ${IMAGE_CLASSES}

I think that it is because inherit takes effect before append?

Another way to fix the problem is:
IMAGE_CLASSES ?= ""
inherit image_types ${IMAGE_CLASSES}

But it seems that we need another name for IMAGE_CLASSES, for example
IMAGE_CLASSES_EXTRA, and also need update the doc, which would make it
complicated.

(From OE-Core rev: c225613d997ef40dedf64b880b5ca4ae89adfded)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>