Add some basic fat32 code

sw/kernel/filesystems/fs.h

@@ -0,0 +1,19 @@
+#ifndef _FS_H
+#define _FS_H
+
+#include <stdint.h>
+
+/* syscalls for files */
+int8_t file_read(int8_t fd, void* buf, int8_t nbytes);
+int8_t file_write(int8_t fd, const void* buf, int8_t nbytes);
+int8_t file_open(const uint8_t* filename);
+int8_t file_close(int8_t fd);
+
+/* syscalls for directories */
+int8_t directory_read(int8_t fd, void* buf, int8_t nbytes);
+int8_t directory_write(int8_t fd, const void* buf, int8_t nbytes);
+int8_t directory_open(const uint8_t* filename);
+int8_t directory_close(int8_t fd);
+
+
+#endif