postmarketos-initramfs: prevent busyloop in syslogd (MR 6013)

when stdin is /dev/null (e.g. when console=null is on the kernel
cmdline) syslogd will call read() over and over and never handle the
SIGTERM we send it, pinning a CPU core on every boot.

Hooking up syslogd std I/O to /dev/zero prevents this.

Co-authored-by: Richard Acayan <mailingradian@gmail.com>
Co-authored-by: Clayton Craft <clayton@craftyguy.net>
[ci:skip-build]: already built successfully in CI