motdgen: do not share a staged file

For the same reason as in coreos#39
(copying the phrasing below):

With the `staged` file shared, there would be potential for two or
more processes executing `motdgen` to write to it resulting in
corrupted output, or the error in the `cat` command due to missing
file reported in coreos#35 (comment). Currently, this is not a problem with
motdgen, but could be if `motdgen` were invoked by something like the
udev rules that invoke `issuegen`.

Instead, write the intermediate output to a variable before writing
to the final output file. This ensures only valid output is written
to the issue file shown to the terminal.

Additionally, perform code tidyups similar to those done for
`issuegen` in coreos#40.

usr/libexec/console-login-helper-messages/motdgen

@@ -12,32 +12,28 @@
 set -e
 
 PKG_NAME=console-login-helper-messages
-MOTD_DIR_PUBLIC=motd.d
-# Should only be read by this script.
-MOTD_DIR_PRIVATE="${PKG_NAME}/motd.d"
+MOTD_SNIPPETS_PATH="${PKG_NAME}/motd.d"
+ETC_SNIPPETS="/etc/${MOTD_SNIPPETS_PATH}"
+RUN_SNIPPETS="/run/${MOTD_SNIPPETS_PATH}"
+USR_LIB_SNIPPETS="/usr/lib/${MOTD_SNIPPETS_PATH}"
 
-staged="/run/${PKG_NAME}/40_${PKG_NAME}.motd.staged"
-# Pick 40 as an index as other files can order around it easily.
-generated="/run/motd.d/40_${PKG_NAME}.motd"
+# Parts of this script write to the `${RUN_SNIPPETS}` directory,
+# make sure it is created upfront.
+mkdir -p "${RUN_SNIPPETS}"
 
-mkdir -p "/run/${MOTD_DIR_PRIVATE}"
-mkdir -p "/run/${MOTD_DIR_PUBLIC}"
-rm -f "${generated}"
 
+# Write distro release information to MOTD, in distro-specific color.
 source /usr/lib/os-release
-echo -e "\e[${ANSI_COLOR}m${PRETTY_NAME}\e[39m" > "/run/${MOTD_DIR_PRIVATE}/21_os_release.motd"
-
-# Generate a motd from files found in the private (package-specific) directories,
-# and place the motd in a public directory.
-if [[ -d "/etc/${MOTD_DIR_PRIVATE}" ]]; then
-	cat /etc/${MOTD_DIR_PRIVATE}/* 2>/dev/null >> "${staged}" || true
-fi
-if [[ -d /run/"${MOTD_DIR_PRIVATE}" ]]; then
-	cat /run/${MOTD_DIR_PRIVATE}/* 2>/dev/null >> "${staged}" || true
-fi
-if [[ -d /usr/lib/"${MOTD_DIR_PRIVATE}" ]]; then
-	cat /usr/lib/${MOTD_DIR_PRIVATE}/* 2>/dev/null >> "${staged}" || true
-fi
-
-cat "${staged}" > "${generated}"
-rm -rf "${staged}"
+echo -e "\e[${ANSI_COLOR}m${PRETTY_NAME}\e[39m" > "${RUN_SNIPPETS}/21_os_release.motd"
+
+
+# Generate a final motd from compiling the snippets.
+# Pick 40 as a prefix as other files can order around it easily.
+generated="/run/motd.d/40_${PKG_NAME}.motd"
+generated_string=''
+# Hack around files potentially not existing in the below paths with `|| true`.
+generated_string+=$(cat ${ETC_SNIPPETS}/* 2>/dev/null || true)
+generated_string+=$(cat ${RUN_SNIPPETS}/* 2>/dev/null || true)
+generated_string+=$(cat ${USR_LIB_SNIPPETS}/* 2>/dev/null || true)
+
+echo "${generated_string}" > "${generated}"