volatile-binds: Calculate the name of the /var/lib service

volatile-binds: Calculate the name of the /var/lib service

By default, /var/lib is bind mounted on /var/volatile/lib. If this is
the case, the recipe adds conditions on systemd-random-seed in the
service file mounting it. But as the VOLATILE_BINDS may be modified,
/var/lib may be mounted elsewhere, for example in /persistent/var/lib.
In this case, the conditions are not set because the service file name
does not match expected one.
This patch automatically records the name of the service mounting
/var/lib, if any, in order to set the condition in the appropriate file.

Signed-off-by: Stéphane Veyret <sveyret@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>