Renamed cache_size -> buffer_size

This makes littlefs's usage of the term "cache" an entirely internal
concept and hopefully avoids some confusion about the usefulness of
throwing RAM > block_size at these buffers.

The term cache isn't entirely inaccurate, these buffers do act as
single-line caches, however more often the term cache is used to
describe multi-line caches. Maybe this will be added in littlefs's
future, but the code-size cost makes this change not worth the overhead
at the moment.

.travis.yml

@@ -50,7 +50,7 @@ _: &test-no-intrinsics
 _: &test-no-inline
   - make test TFLAGS+="-nrk -DLFS_INLINE_MAX=0"
 _: &test-byte-writes
-  - make test TFLAGS+="-nrk -DLFS_READ_SIZE=1 -DLFS_CACHE_SIZE=1"
+  - make test TFLAGS+="-nrk -DLFS_READ_SIZE=1 -DLFS_BUFFER_SIZE=1"
 _: &test-block-cycles
   - make test TFLAGS+="-nrk -DLFS_BLOCK_CYCLES=1"
 _: &test-odd-block-count
@@ -220,7 +220,7 @@ jobs:
             -DLFS_BLOCK_SIZE=512
             -DLFS_BLOCK_COUNT=1024
             -DLFS_BLOCK_CYCLES=-1
-            -DLFS_CACHE_SIZE=64
+            -DLFS_BUFFER_SIZE=64
             -DLFS_LOOKAHEAD_SIZE=16
             -DLFS_NO_ASSERT -DLFS_NO_DEBUG -DLFS_NO_WARN -DLFS_NO_ERROR"
     if: branch !~ -prefix$