Cleaned up block allocator

Removed scanning for stride
- Adds complexity with questionable benefit
- Can be added as an optimization later

Fixed handling around device boundaries and where lookahead may not be a
factor of the device size (consider small devices with only a few
blocks)

Added support for configuration with optional dynamic memory as found in
the caching configuration

tests/template.fmt

@@ -70,6 +70,10 @@ uintmax_t res;
 #define LFS_BLOCK_COUNT 1024
 #endif
 
+#ifndef LFS_LOOKAHEAD
+#define LFS_LOOKAHEAD 128
+#endif
+
 const struct lfs_config cfg = {{
     .context = &bd,
     .read  = &lfs_emubd_read,
@@ -81,6 +85,7 @@ const struct lfs_config cfg = {{
     .prog_size   = LFS_PROG_SIZE,
     .block_size  = LFS_BLOCK_SIZE,
     .block_count = LFS_BLOCK_COUNT,
+    .lookahead   = LFS_LOOKAHEAD,
 }};