sysvinit: bootlogd: Don't run savelog if it's not installed

sysvinit: bootlogd: Don't run savelog if it's not installed

bootlogd's default log rotation code on stop requires `savelog`, which
is in debianutils, which may not be installed.  If it's not installed,
don't try to perform the log rotation.

That is: in the affected code block, `savelog` is what is responsible
for creating "boot.0".  When `savelog` doesn't exist, an error message
gets printed on bootup to the effect of "mv: can't find boot.0".

(From OE-Core rev: 5c22973e1bf76615bcf57984ac7a30cf7d0766df)

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>