lib/string: Add standard strstr() function

Adding implementation of standard library strstr()

See https://review.coreboot.org/c/coreboot/+/43741 for context.

Change-Id: I63e26e98ed2dd15542f81c0a3a5e353bb93b7350
Signed-off-by: jbk@chromium.org
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44085
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/include/string.h

@@ -29,6 +29,7 @@ int strcmp(const char *s1, const char *s2);
 int strncmp(const char *s1, const char *s2, int maxlen);
 int strspn(const char *str, const char *spn);
 int strcspn(const char *str, const char *spn);
+char *strstr(const char *haystack, const char *needle);
 char *strtok_r(char *str, const char *delim, char **ptr);
 char *strtok(char *str, const char *delim);
 long atol(const char *str);

src/lib/string.c

@@ -163,6 +163,16 @@ int strcspn(const char *str, const char *spn)
 	return ret;
 }
 
+char *strstr(const char *haystack, const char *needle)
+{
+	size_t needle_len = strlen(needle);
+	for (; *haystack; haystack++) {
+		if (!strncmp(haystack, needle, needle_len))
+			return (char *)haystack;
+	}
+	return NULL;
+}
+
 char *strtok_r(char *str, const char *delim, char **ptr)
 {
 	char *start;