file/fat: add utime function

This emulates the traditional utime function from UNIX clones to allow
for manual updates of the modification timestamp on files and directories.

This should only prove useful for non-native targets as those usually
have a libc version of utime.

Change-Id: Iea8a1d328e78b92c400d3354ee80689c7cf53af8

firmware/include/file.h

@@ -85,6 +85,9 @@ int fdprintf(int fildes, const char *fmt, ...) ATTRIBUTE_PRINTF(2, 3);
 #ifndef rename
 #define rename          FS_PREFIX(rename)
 #endif
+#ifndef utime
+#define utime           FS_PREFIX(utime)
+#endif
 #ifndef filesize
 #define filesize        FS_PREFIX(filesize)
 #endif