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

lfs_config.h

@@ -23,11 +23,6 @@ typedef uint32_t lfs_block_t;
 #define LFS_NAME_MAX 255
 #endif
 
-// Lookahead distance
-#ifndef LFS_CFG_LOOKAHEAD
-#define LFS_CFG_LOOKAHEAD 128
-#endif
-
 // Logging operations
 #include <stdio.h>
 #define LFS_ERROR(fmt, ...) printf("lfs error: " fmt "\n", __VA_ARGS__)