package/linuxptp: make initscript options configurable

package/linuxptp: make initscript options configurable

The "package/busybox/S01sysklogd" is taken as template to change the
init script of the linuxptp daemon. The init script is split two parts
because there are 2 daemons (ptp4l and phc2sys).

Let the user supply its own options in /etc/default/ptp4l and
/etc/default/phc2sys.

This patch also fixes an issue with the creation of the pid file that is
needed to properly stop the daemon again.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Tested-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>