Add fat12 lib

fat12.h

@@ -0,0 +1,15 @@
+#pragma once
+
+/*
+ * Arguments:
+ * - fat_addr: 1 KiB
+ * - rootdir_addr: 3.5 KiB
+ */
+int fat12_init(void* fat_addr, void* rootdir_addr);
+
+/*
+ * Returns:
+ *  -4 if file is not found
+ *  -5 if there is a disk error
+ */ 
+int fat12_readfile(const char* name, void* addr);