commonlib/bsd: Add strlen() and strnlen() functions

Add strlen() and strnlen() to commonlib/bsd by rewriting them from
scratch, and remove the same functions from coreboot and libpayload.

Note that in the existing libpayload implementation, these functions
return 0 for NULL strings. Given that POSIX doesn't require the NULL
check and that other major libc implementations (e.g. glibc [1]) don't
seem to do that, the new functions also don't perform the NULL check.

[1] https://github.com/bminor/glibc/blob/master/sysdeps/i386/strlen.c

Change-Id: I1203ec9affabe493bd14b46662d212b08240cced
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83830
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/commonlib/bsd/string.c

@@ -2,6 +2,26 @@
 
 #include <commonlib/bsd/string.h>
 #include <ctype.h>
+#include <stddef.h>
+
+size_t strlen(const char *str)
+{
+	const char *ptr = str;
+
+	while (*ptr++)
+		;
+	return ptr - str - 1;
+}
+
+size_t strnlen(const char *str, size_t maxlen)
+{
+	const char *ptr = str;
+	const char *end = str + maxlen + 1;
+
+	while (*ptr++ && ptr < end)
+		;
+	return ptr - str - 1;
+}
 
 unsigned int skip_atoi(char **ptr)
 {