libpayload/libc: Correct strlcat return value

The documented return value for strlcat is horribly wrong, as is the
return value itself. It should not return the number of appended bytes,
but rather the length of the concatenated string. From the man page:

    The strlcpy() and strlcat() functions return the total length of the
    string they tried to create. For strlcpy() that means the length of
    src. For strlcat() that means the initial length of dst plus the
    length of src. While this may seem somewhat confusing, it was done
    to make truncation detection simple.

This change is more likely to fix existing code than break it, since
anyone who uses the return value of strlcat will almost certainly rely
on the standard behaviour rather than investigate coreboot's source code
to see that we have a quirky version.

Change-Id: I4421305af85bce88d12d6fdc2eea6807ccdcf449
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33787
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/libc/string.c

@@ -249,7 +249,7 @@ char *strncat(char *d, const char *s, size_t n)
  * @param d The destination string.
  * @param s The source string.
  * @param n d will have at most n-1 characters (plus NUL) after invocation.
- * @return A pointer to the destination string.
+ * @return The total length of the concatenated string.
  */
 size_t strlcat(char *d, const char *s, size_t n)
 {
@@ -264,7 +264,7 @@ size_t strlcat(char *d, const char *s, size_t n)
 		p[i] = s[i];
 
 	p[i] = '\0';
-	return max;
+	return sl + dl;
 }
 
 /**