lib.sh: add fe_ which is fx_ but err on find

In the mk script, we need fx_ to not return errors on the
find command, since it's searching a bunch of directories
where some of them may not exist.

All other instances where fx_ is used, must return an error
if the directory being searched doesn't exist.

For this, fe_() is introduced, which does the same as fx_
but with this much stricter check.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/lib.sh

@@ -128,11 +128,22 @@ setvars()
 	printf "%s\n" "${_setvars% }"
 }
 
+fe_()
+{
+	find_ex "x_" "$@"
+}
+
 fx_()
 {
+	find_ex "" "$@"
+}
+
+find_ex()
+{
+	errx="$1" && shift 1
 	fd="`mktemp`"
 	xx="$1" && shift 1
-	find "$@" | sort > "$fd" || $err "!find $(echo "$@") > \"$fd\""
+	$errx find "$@" | sort > "$fd" || $err "!find $(echo "$@") > \"$fd\""
 	while read -r fx; do
 		"$xx" "$fx" || break; :
 	done < "$fd"