[yocto] populate_sdk in image class

[Paulo Neves]

A long time ago the image class got an inherit to the populate_sdk*
class which brings a lot of dependencies on python modules.

Is it strictly necessary that to build an image, i need to satisfy
nativesdk-libgcc among others? Is there not a performance and
complexity impact?

I never needed to use the nativesdk separately as "the primary way of
running builds within the extensible SDK is to use the 'devtool'
command" (quote from SHA1 545673). Should not this class inheritance
be moved into devtool or at least make the dependencies be triggered
with the do_populate_sdk* task?