image-buildinfo: Improve and extend to SDK coverage too

Rename the default file from "build" to "buildinfo" since this is more
obvious to anyone looking in an image.

Add SDK_BUILDINFO_FILE and allow the same information to be written into
SDK images. This will be useful for buildtools-tarball and uninative-tarball.

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