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.

tests/test_orphans.toml

@@ -59,7 +59,7 @@ code = '''
 [[case]] # reentrant testing for orphans, basically just spam mkdir/remove
 reentrant = true
 # TODO fix this case, caused by non-DAG trees
-if = '!(DEPTH == 3 && LFS_CACHE_SIZE != 64)'
+if = '!(DEPTH == 3 && LFS_BUFFER_SIZE != 64)'
 define = [
     {FILES=6,  DEPTH=1, CYCLES=20},
     {FILES=26, DEPTH=1, CYCLES=20},