Added initial superblock definition

Really started working out how the internal structure of the driver
will be organized. There are a few hazy lines between the intended
data structures with the goal of code reuse, so the function boundaries
may end up a bit weird.

lfs_config.h

@@ -42,4 +42,7 @@ static inline lfs_word_t lfs_npw2(lfs_word_t a) {
     return 32 - __builtin_clz(a-1);
 }
 
+// Attributes
+#define lfs_disk_struct struct __attribute__((packed))
+
 #endif