string: Add strcspn()

Add an implementation of strcspn() which returns the number of initial
characters that do not match any in a rejection list.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/string.c

@@ -286,6 +286,30 @@ size_t strnlen(const char * s, size_t count)
 }
 #endif
 
+#ifndef __HAVE_ARCH_STRCSPN
+/**
+ * strcspn - Calculate the length of the initial substring of @s which does
+ * not contain letters in @reject
+ * @s: The string to be searched
+ * @reject: The string to avoid
+ */
+size_t strcspn(const char *s, const char *reject)
+{
+	const char *p;
+	const char *r;
+	size_t count = 0;
+
+	for (p = s; *p != '\0'; ++p) {
+		for (r = reject; *r != '\0'; ++r) {
+			if (*p == *r)
+				return count;
+		}
+		++count;
+	}
+	return count;
+}
+#endif
+
 #ifndef __HAVE_ARCH_STRDUP
 char * strdup(const char *s)
 {