Lower kernel log level to 4 before exiting initramfs

Closes: coreos/fedora-coreos-tracker#1244

A lot going on for this simple service.  See the tracker issue above
for more info, but briefly:

Anaconda has historically injected `quiet` into the kernel command
line in many cases, and this suppresses *both* kernel and systemd
output.  On computers in general, but particularly many bare metal
servers, the serial console can be slow.  This can cause reliability
issues.

However for servers, we usually *do* want to see informational
output when they boot.  For example, today the kernel "mitigations"
information for hardware vulnerabilities is output.

This change is a compromise; we boot up at the kernel's default
verbosity level (which for Fedora and derivatives is the upstream 7),
but switch to 4 before exiting the initramfs.  At that point, we've
gotten most of the boot time output, and our initramfs is not
extremely performance sensitive right now.

overlay.d/05core/usr/lib/dracut/modules.d/10coreos-printk/coreos-printk-quiet.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=CoreOS: Quiet printk after initramfs setup
+Documentation=https://github.com/coreos/fedora-coreos-tracker/issues/1244
+ConditionPathExists=/etc/initrd-release
+After=initrd.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+# Change the kernel to default to warning level
+ExecStart=/bin/sh -c 'echo 4 > /proc/sys/kernel/printk'

overlay.d/05core/usr/lib/dracut/modules.d/10coreos-printk/module-setup.sh

@@ -0,0 +1,25 @@
+# See https://issues.redhat.com/browse/RFE-2977 and
+# https://github.com/coreos/fedora-coreos-tracker/issues/1244
+# Basically, the current Fedora kernel log level is "7" which is quite
+# high.  We could change the kernel, and it's tempting to do so but
+# there's some nontrivial potential fallout from that.
+# As a compromise, we keep the default log level for the initramfs,
+# but quiet it before switching to the real root.  This will ensure
+# we'll get most useful logging of bootup configuration.
+
+check() {
+    return 0
+}
+
+install_unit() {
+    unit="$1"; shift
+    target="$1"; shift
+    inst_simple "$moddir/$unit" "$systemdsystemunitdir/$unit"
+    # note we `|| exit 1` here so we error out if e.g. the units are missing
+    # see https://github.com/coreos/fedora-coreos-config/issues/799
+    systemctl -q --root="$initdir" add-requires "$target" "$unit" || exit 1
+}
+
+install() {
+    install_unit coreos-printk-quiet.service initrd.target
+}

tests/kola/logging/printk.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+# kola: { "exclusive": false }
+
+# Verify fix for https://github.com/coreos/fedora-coreos-tracker/issues/1244
+
+set -xeuo pipefail
+
+. $KOLA_EXT_DATA/commonlib.sh
+
+printk=$(cat /proc/sys/kernel/printk)
+
+if ! [[ "$printk" =~ ^4 ]]; then
+    fatal "printk: expected console log level 4, found ${printk}"
+fi
+
+ok "Found expected printk value "