lib: Add a way to find the postiion of a trailing number

At present it is not possible to find out which part of the string is the
number part and which is before it. Add a new variant which provides this
feature, so we can separate the two in the caller.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/str_ut.c b/test/str_ut.c
index 058b359..5a84434 100644
--- a/test/str_ut.c
+++ b/test/str_ut.c
@@ -244,7 +244,9 @@
 
 static int str_trailing(struct unit_test_state *uts)
 {
-	char str1[] = "abc123def";
+	const char str1[] = "abc123def";
+	const char str2[] = "abc123def456";
+	const char *end;
 
 	ut_asserteq(-1, trailing_strtol(""));
 	ut_asserteq(-1, trailing_strtol("123"));
@@ -259,6 +261,15 @@
 
 	ut_asserteq(3, trailing_strtol("a3"));
 
+	ut_asserteq(123, trailing_strtoln_end(str1, str1 + 6, &end));
+	ut_asserteq(3, end - str1);
+
+	ut_asserteq(-1, trailing_strtoln_end(str1, str1 + 7, &end));
+	ut_asserteq(7, end - str1);
+
+	ut_asserteq(456, trailing_strtoln_end(str2, NULL, &end));
+	ut_asserteq(9, end - str2);
+
 	return 0;
 }
 STR_TEST(str_trailing, 0);