use newlib API for standard functions

[d-a-v]

From @devyte, we should use newlib's API

#ifndef lwip_itoa
/* This can be #defined to itoa() or snprintf(result, bufsize, "%d", number) depending on your platform */
void  lwip_itoa(char* result, size_t bufsize, int number);
#endif
#ifndef lwip_strnicmp
/* This can be #defined to strnicmp() or strncasecmp() depending on your platform */
int   lwip_strnicmp(const char* str1, const char* str2, size_t len);
#endif
#ifndef lwip_stricmp
/* This can be #defined to stricmp() or strcasecmp() depending on your platform */
int   lwip_stricmp(const char* str1, const char* str2);
#endif
#ifndef lwip_strnstr
/* This can be #defined to strnstr() depending on your platform */
char* lwip_strnstr(const char* buffer, const char* token, size_t n);
#endif