Changed block_cycles disable from 0 to -1

As it is now, block_cycles = 0 disables wear leveling. This was a
mistake as 0 is the "default" value for several other config options.
It's even worse when migrating from v1 as it's easy to miss the addition
of block_cycles and end up with a filesystem that is not actually
wear-leveling.

Clearly, block_cycles = 0 should do anything but disable wear-leveling.

Here, I've changed block_cycles = 0 to assert. Forcing users to set a
value for block_cycles (500 is suggested). block_cycles can be set to -1
to explicitly disable wear leveling if desired.

lfs.h

@@ -190,9 +190,12 @@ struct lfs_config {
     // Number of erasable blocks on the device.
     lfs_size_t block_count;
 
-    // Number of erase cycles before we should move data to another block.
-    // May be zero, in which case no block-level wear-leveling is performed.
-    uint32_t block_cycles;
+    // Number of erase cycles before we should move logs to another block.
+    // Suggested values are in the range 100-1000, with large values having
+    // better performance at the cost of less consistent wear distribution.
+    //
+    // Set to -1 to disable block-level wear-leveling.
+    int32_t block_cycles;
 
     // Size of block caches. Each cache buffers a portion of a block in RAM.
     // The littlefs needs a read cache, a program cache, and one additional