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>

payloads/libpayload/libc/string.c

@@ -35,50 +35,6 @@
 #include <limits.h>
 #include <errno.h>
 
-/**
- * Calculate the length of a fixed-size string.
- *
- * @param str The input string.
- * @param maxlen Return at most maxlen characters as length of the string.
- * @return The length of the string, not including the final NUL character.
- * 	   The maximum length returned is maxlen.
- */
-size_t strnlen(const char *str, size_t maxlen)
-{
-	size_t len = 0;
-
-	/* NULL and empty strings have length 0. */
-	if (!str)
-		return 0;
-
-	/* Loop until we find a NUL character, or maxlen is reached. */
-	while ((*str++ != '\0') && (len < maxlen))
-		len++;
-
-	return len;
-}
-
-/**
- * Calculate the length of a string.
- *
- * @param str The input string.
- * @return The length of the string, not including the final NUL character.
- */
-size_t strlen(const char *str)
-{
-	size_t len = 0;
-
-	/* NULL and empty strings have length 0. */
-	if (!str)
-		return 0;
-
-	/* Loop until we find a NUL character. */
-	while (*str++ != '\0')
-		len++;
-
-	return len;
-}
-
 /**
  * Compare two strings.
  *