build_library: Add mangle script for zfs sysext

There are two challenges with the sysext: it needs config files in /etc
and it needs udev rules for mounting during boot to work. The etc files
are placed in the standard flatcar etc overlay path but the overlay is
mounted from the initrd. So instead, we create a tmpfiles.d rule that
symlinks the best important files over. For the udev issue, we create a
drop-in in /etc that ensures udev runs after systemd-sysext.

We also can't rely on systemd presets to work, so instead parse the
preset file and statically create the service dependencies. For the
primary zfs.target we rely on an Upholds entry. Users can still disabled
unwanted services if they want.

We also removed unnecessary files:
- development files
- initramfs related scripts

Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

build_library/sysext_mangle_flatcar-zfs

@@ -0,0 +1,43 @@
+#!/bin/bash
+
+set -euo pipefail
+rootfs="${1}"
+
+pushd "${rootfs}"
+
+rm -rf ./usr/{lib/debug/,lib64/cmake/,include/}
+rm -rf ./usr/lib/dracut/
+rm -rf ./usr/share/initramfs-tools
+rm -rf ./usr/src
+
+mkdir -p ./usr/share/zfs/etc
+rm -rf ./etc/{csh.env,environment.d/,profile.env}
+cp -a ./etc/. ./usr/share/zfs/etc/
+
+pushd ./usr/lib/systemd/system
+while read cmd unit; do
+  if [ "$cmd" = enable ]; then
+    target=$(awk -F= '/WantedBy/ { print $2 }' $unit)
+    mkdir -p "${target}.wants"
+    ln -svr "${unit}" "${target}".wants/
+  fi
+done < <(grep -v '^#' "${rootfs}"/usr/lib/systemd/system-preset/50-zfs.preset)
+mkdir -p "multi-user.target.d"
+{ echo "[Unit]"; echo "Upholds=zfs.target"; } > "multi-user.target.d/10-zfs.conf"
+popd
+
+mkdir -p ./usr/lib/tmpfiles.d
+cat <<EOF >./usr/lib/tmpfiles.d/10-zfs.conf
+d  /etc/zfs                                                 0755  root  root  -  -
+L  /etc/zfs/zed.d                                           -     -     -     -  /usr/share/zfs/etc/zfs/zed.d
+L  /etc/zfs/zfs-functions                                   -     -     -     -  /usr/share/zfs/etc/zfs/zfs-functions
+L  /etc/zfs/zpool.d                                         -     -     -     -  /usr/share/zfs/etc/zfs/zpool.d
+C  /etc/systemd/system/systemd-udevd.service.d/10-zfs.conf  -     -     -     -  /usr/lib/systemd/system/systemd-udevd.service.d/10-zfs.conf
+EOF
+
+mkdir -p ./usr/lib/systemd/system/systemd-udevd.service.d
+cat <<EOF >./usr/lib/systemd/system/systemd-udevd.service.d/10-zfs.conf
+[Unit]
+After=systemd-sysext.service
+EOF
+popd