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/include/string.h

@@ -13,8 +13,6 @@ int memcmp(const void *s1, const void *s2, size_t n);
 void *memchr(const void *s, int c, size_t n);
 char *strdup(const char *s);
 char *strconcat(const char *s1, const char *s2);
-size_t strnlen(const char *src, size_t max);
-size_t strlen(const char *src);
 char *strchr(const char *s, int c);
 char *strncpy(char *to, const char *from, size_t count);
 char *strcpy(char *dst, const char *src);