oeqa/selftest: Add lower layer test for overlayfs-etc

oeqa/selftest: Add lower layer test for overlayfs-etc

Place a test file on the /etc by means of overlayfs-user recipe.
Perform QA checks to make sure that:
- When lower layer is exposed, that it's read-only to avoid undefined behavior
- By default lower layer is not exposed

Signed-off-by: Vyacheslav Yurkov <v.yurkov@precitec.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>