samples: sockets: Add e1000 and stellaris ethernet drivers overlays

Add qemu ethernet driver overlays for big_http_download and
dumb_http_server samples, which are default integration samples/
tests for TCP stack. This change follows up after similar addition
of such overlays to other samples (and smsc911x overlay addition
to these samples).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>