overlayfs: add docs about skipping QA check & service dependencies

overlayfs: add docs about skipping QA check & service dependencies

Add some documentation about skipping the QA check related to missing
fstab entries or mount units for base mount points where the overlayfs
is mounted from.

Also add a short paragraph about adding a systemd unit dependency to
services in recipes, so that they are started only after the overlayfs
is mounted and ready.

No functional change.

Signed-off-by: Claudius Heine <ch@denx.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>