busybox.inc: Avoid error when SYSLOG is not enabled

Add -f to rm of ${D}${sysconfdir}/syslog-startup.conf.${BPN} so as to
not error out when the busybox config used does not have SYSLOG enabled
and DISTRO_FEATURES does not contain sysvinit.

Signed-off-by: Florin Sarbu <florin.sarbu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>