Fix build iso routine to be able to deal with optional config keys

Earlier this year, we had to adapt the routine to be able to define the net devices and
the interfaces diffently than the default names (net0, ...). It worked at the time
because we declared those in the $(hostname).yaml configuration file. But now, trying to
use the default values (without specifying those new entries), the routine does not
work. The cli j2 is not happy about non declared template values (IPXE_NET, ...) [1]

This fixes it by using temporary templates which gets adapted through sed to fill-in the
holes. Once done, the temporary templates are cleaned up.

[1]
```
jinja2.exceptions.UndefinedError: 'IPXE_NET' is undefined
```

Refs. sysadm-environment#5519

configs/build_iso.sh

@@ -18,40 +18,62 @@ if ! [ -f "${CONFIG}" ]; then
     exit 2
 fi
 
-j2 -f yaml "${BASEDIR}/configs/template.ipxe.j2" "${CONFIG}" >"${IPXE_SCRIPT}"
+TMPDIR="$(mktemp -d)"
+trap 'rm -rf -- "${TMPDIR}"' EXIT
 
-# If not provided in the hostname configuration, fallback to the net0
-sed -i s'/{{IPXE_NET}}/net0/g' "${IPXE_SCRIPT}"
+TEMPLATE_IPXE_TMP="${TMPDIR}/template.ipxe.j2"
+cp ${BASEDIR}/configs/template.ipxe.j2 $TEMPLATE_IPXE_TMP
 
+# Set optional value IPXE_NET directly if not provided in the yaml
+# (missing templating variable will make the j2 call fail otherwise)
+grep "IPXE_NET" $CONFIG >/dev/null || sed -i s'/{{IPXE_NET}}/net0/g' "${TEMPLATE_IPXE_TMP}"
+
+j2 -f yaml "${TEMPLATE_IPXE_TMP}" "${CONFIG}" > "${IPXE_SCRIPT}"
 
 make -C "${BASEDIR}/src" -j8 "${IPXE_TARGET}" EMBED="${IPXE_SCRIPT}" DEBUG=eth_slow NO_WERROR=1
 mv "${BASEDIR}/src/${IPXE_TARGET}" "${BASEDIR}/configs/${HOSTNAME}.iso"
 
 echo "built iso image ${BASEDIR}/configs/${HOSTNAME}.iso" 1>&2
 
-[[ -e "${PASSWORDS_FILE}" ]] && PASSWORD=$(awk -F: "\$1 == \"${HOSTNAME}\" { print \$2 }" ${PASSWORDS_FILE})
+[[ -e "${PASSWORDS_FILE}" ]] && \
+    PASSWORD=$(awk -F: "\$1 == \"${HOSTNAME}\" { print \$2 }" ${PASSWORDS_FILE})
 
 if [ -z "$PASSWORD" ]; then
     PASSWORD="$(xkcdpass -d- -a "${HOSTNAME:0:4}")"
 
     echo "${HOSTNAME}:${PASSWORD}" >>${PASSWORDS_FILE}
-
 fi
 
 CRYPTED_PASSWORD="$(echo "${PASSWORD}" | mkpasswd -s)"
 
-PRESEED_FILE="${BASEDIR}/configs/preseeding/${HOSTNAME}.txt"
-env CRYPTED_PASSWORD=$CRYPTED_PASSWORD j2 -f yaml -e env "${BASEDIR}/configs/preseeding/preseed.txt.j2" "${CONFIG}" >"${PRESEED_FILE}"
+PRESEED_DIR="${BASEDIR}/configs/preseeding"
+PRESEED_FILE="${PRESEED_DIR}/${HOSTNAME}.txt"
 
-# If not provided in the hostname configuration, fallback to the default interface
-sed -i s'/{{PRESEED_INTERFACE0}}/ens10f0np0/g' "${PRESEED_FILE}"
-sed -i s'/{{PRESEED_INTERFACE1}}/ens10f0np1/g' "${PRESEED_FILE}"
+TEMPLATE_PRESEED_TMP="${TMPDIR}/preseed.txt.j2"
+cp ${PRESEED_DIR}/preseed.txt.j2 $TEMPLATE_PRESEED_TMP
+
+# If not provided in the hostname configuration, fallback to the default interfaces
+# (required to be done prior to the j2 call otherwise, it will fail)
+grep "PRESEED_INTERFACE0" $CONFIG >/dev/null || \
+    sed -i s'/{{PRESEED_INTERFACE0}}/ens10f0np0/g' "${TEMPLATE_PRESEED_TMP}"
+grep "PRESEED_INTERFACE1" $CONFIG >/dev/null || \
+    sed -i s'/{{PRESEED_INTERFACE1}}/ens10f0np1/g' "${TEMPLATE_PRESEED_TMP}"
 
-FINISH_INSTALL="${BASEDIR}/configs/preseeding/finish_install/${HOSTNAME}.sh"
-j2 -f yaml -e env "${BASEDIR}/configs/preseeding/finish_install/finish_install.sh.j2" "${CONFIG}" >"${FINISH_INSTALL}"
+env CRYPTED_PASSWORD=$CRYPTED_PASSWORD \
+    j2 -f yaml -e env "${TEMPLATE_PRESEED_TMP}" "${CONFIG}" > "${PRESEED_FILE}"
+
+FINISH_INSTALL_DIR="${PRESEED_DIR}/finish_install"
+FINISH_INSTALL="${FINISH_INSTALL_DIR}/${HOSTNAME}.sh"
+
+TEMPLATE_FINISH_INSTALL_TMP="${TMPDIR}/finish_install.sh.j2"
+cp ${FINISH_INSTALL_DIR}/finish_install.sh.j2 $TEMPLATE_FINISH_INSTALL_TMP
 
 # If not provided in the hostname configuration, fallback to the default interface
-sed -i s'/{{PRESEED_INTERFACE0}}/ens10f0np0/g' "${FINISH_INSTALL}"
-sed -i s'/{{PRESEED_INTERFACE1}}/ens10f0np1/g' "${FINISH_INSTALL}"
+grep "PRESEED_INTERFACE0" $CONFIG >/dev/null || \
+    sed -i s'/{{PRESEED_INTERFACE0}}/ens10f0np0/g' "${TEMPLATE_FINISH_INSTALL_TMP}"
+grep "PRESEED_INTERFACE1" $CONFIG >/dev/null || \
+    sed -i s'/{{PRESEED_INTERFACE1}}/ens10f0np1/g' "${TEMPLATE_FINISH_INSTALL_TMP}"
+
+j2 -f yaml -e env "${TEMPLATE_FINISH_INSTALL_TMP}" "${CONFIG}" >"${FINISH_INSTALL}"
 
 echo "Generated preseeding config in ${PRESEED_FILE} and ${FINISH_INSTALL}." 1>&2