EXTRA_IMAGEDEPENDS is used to ensure things like qemu-native are built and these...

EXTRA_IMAGEDEPENDS is used to ensure things like qemu-native are built and these are not strictly dependencies of the do_rootfs task. This patch moves them to the do_build task allowing bitbake a little more flexibility about when things need to get built by.

I noticed this when qemu-native failed to build and a rootfs was not generated
which is not the intended behaviour.

Also update the syntax to use appendVarFlag instead of get/setVarFlag

(From OE-Core rev: fa13e83ec3f91dce866ac212e91b62db24b6486d)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>