Introduced cache_size as alternative to hardware read/write sizes

The introduction of an explicit cache_size configuration allows
customization of the cache buffers independently from the hardware
read/write sizes.

This has been one of littlefs's main handicaps. Without a distinction
between cache units and hardware limitations, littlefs isn't able to
read or program _less_ than the cache size. This leads to the
counter-intuitive case where larger cache sizes can actually be harmful,
since larger read/prog sizes require sending more data over the bus if
we're only accessing a small set of data (for example the CTZ skip-list
traversal).

This is compounded with metadata logging, since a large program size
limits the number of commits we can write out in a single metadata
block. It really doesn't make sense to link program size + cache
size here.

With a separate cache_size configuration, we can be much smarter about
what we actually read/write from disk.

This also simplifies cache handling a bit. Before there were two
possible cache sizes, but these were rarely used. Note that the
cache_size is NOT written to the superblock and can be freely changed
without breaking backwards compatibility.

.travis.yml

@@ -18,8 +18,8 @@ script:
   - make test QUIET=1
 
   # run tests with a few different configurations
-  - make test QUIET=1 CFLAGS+="-DLFS_READ_SIZE=1      -DLFS_PROG_SIZE=1"
-  - make test QUIET=1 CFLAGS+="-DLFS_READ_SIZE=512    -DLFS_PROG_SIZE=512"
+  - make test QUIET=1 CFLAGS+="-DLFS_READ_SIZE=1      -DLFS_CACHE_SIZE=4"
+  - make test QUIET=1 CFLAGS+="-DLFS_READ_SIZE=512    -DLFS_CACHE_SIZE=512"
   - make test QUIET=1 CFLAGS+="-DLFS_BLOCK_COUNT=1023 -DLFS_LOOKAHEAD=2048"
 
   - make clean test QUIET=1 CFLAGS+="-DLFS_INLINE_MAX=0"