init-storage: Support custom writable paths in /etc

Custom builds of Aimee OS can now specify additional paths under `/etc`
that should be writable.  This is accomplished by populating a file
named `/etc/aimee-os/writable-etc` with a list of paths.  Each line must
indicate the type of file (regular file: `f`, directory: `d`) and the
*relative* path under `/etc`.

overlay/usr/libexec/init-storage

@@ -68,6 +68,26 @@ setup_etc() {
         cp -ca /etc/shadow "${tmpdir}"/shadow || exit
     fi
     mount -o bind "${tmpdir}"/shadow /etc/shadow || exit
+    if [ -f /etc/aimee-os/writable-etc ]; then
+        while read type path; do
+            if [ ! -e "${tmpdir}/${path}" ]; then
+                if [ -e /etc/"${path}" ]; then
+                    cp -ca /etc/"${path}" "${tmpdir}/${path}"
+                elif [ "${type}" = d ]; then
+                    mkdir -p "${tmpdir}/${path}"
+                elif [ "${type}" = f ]; then
+                    : > "${tmpdir}/${path}"
+                else
+                    printf 'Invalid path type %s for %s\n' \
+                        "${type}" \
+                        "${path}" \
+                        >&2
+                    continue
+                fi
+            fi
+            mount -o bind "${tmpdir}/${path}" /etc/"${path}"
+        done < /etc/aimee-os/writable-etc
+    fi
     umount "${tmpdir}"
 }