[OE-core] inheirt multi bbclass using variable

[Ni Qingliang]

I'm trying to inherit multi bbclass using variable:

IMAGE_EXTRA_CLASSES = "image-timezone image-clearboot image-prd"
inherit ${IMAGE_EXTRA_CLASSES}

and got error:
ERROR: Could not inherit file classes/image-timezone image-clearboot
image-prd.bbclass

why it treat the variable as a single class?
how to resolve the problem?

thanks!


-- 
Yi Qingliang
niqingliang at insigma.com.cn
https://niqingliang2003.wordpress.com