[FS, USER] Debug filesystem, fs syscalls

kernel/fs/debug_fs.c

@@ -0,0 +1,20 @@
+#include <vfs.h>
+#include <debug.h>
+
+size_t debug_write(file_t *file, void *buffer, size_t nbyte, size_t offset)
+{
+  (void)file;
+  (void)offset;
+  debug_putsn(buffer, nbyte);
+  return nbyte;
+}
+
+fs_driver_t debug_fs = {
+  .write = debug_write,
+};
+
+file_t debug_file = {
+  .refs = 1,
+  .type = FS_TTY,
+  .driver = &debug_fs,
+};