[oe] the awkwardness of using core-image.bbclass

[Khem Raj]

>   i know, which is exactly what is so counter-intuitive with the way
> the above is done. you can override those two variables, but not
> CORE_IMAGE_BASE_INSTALL directly, it just seems silly.

why do you want CORE_IMAGE_BASE_INSTALL to be overridable ? unless you
want to construct image of your own
completely from scratch but then why would you want to inherit
properties from core-image class.