kiwibd: Added kiwibd, a lighter-weight variant of emubd

Useful for emulating much larger disks in a file (or in RAM). kiwibd
doesn't have all the features of emubd, but this allows it to prioritize
disk size and speed for benchmarking.

kiwibd still keeps some features useful for benchmarking/emulation:

- Optional erase value emulation, including nor-masking

- Read/prog/erase trackers for measuring bd operations

- Read/prog/erase sleeps for slowing down the simulation to a human
  viewable speed

bd/lfs3_emubd.h

@@ -1,6 +1,10 @@
 /*
- * Emulating block device, wraps filebd and rambd while providing a bunch
- * of hooks for testing littlefs in various conditions.
+ * emubd - High-level emulating block device with many bells and
+ * whistles for testing powerloss, wear, etc.
+ *
+ * Note emubd always backs the block device in RAM. Consider using
+ * kiwibd if you need a block device larger than the available RAM on
+ * the system.
  *
  * Copyright (c) 2022, The littlefs authors.
  * Copyright (c) 2017, Arm Limited. All rights reserved.
@@ -11,8 +15,6 @@
 
 #include "lfs3.h"
 #include "lfs3_util.h"
-#include "bd/lfs3_rambd.h"
-#include "bd/lfs3_filebd.h"
 
 
 // Block device specific tracing
@@ -99,7 +101,8 @@ struct lfs3_emubd_cfg {
     uint32_t seed;
 
     // Path to file to use as a mirror of the disk. This provides a way to view
-    // the current state of the block device.
+    // the current state of the block device, but does not eliminate the RAM
+    // requirement.
     const char *disk_path;
 
     // Artificial delay in nanoseconds, there is no purpose for this other
@@ -157,8 +160,11 @@ typedef struct lfs3_emubd {
 
 // Create an emulating block device using the geometry in lfs3_cfg
 //
-// Note that filebd is used if a path is provided, if path is NULL
-// emubd will use rambd which can be much faster.
+// If disk_path is provided, emubd will mirror the block device in the
+// file. Note this is a write-only mirror intended for introspection,
+// and does not eliminate the RAM requirement.
+//
+// TODO wait, why do we have both disk_path and path here?
 int lfs3_emubd_create(const struct lfs3_cfg *cfg, const char *path);
 int lfs3_emubd_createcfg(const struct lfs3_cfg *cfg, const char *path,
         const struct lfs3_emubd_cfg *bdcfg);
@@ -186,7 +192,7 @@ int lfs3_emubd_erase(const struct lfs3_cfg *cfg, lfs3_block_t block);
 int lfs3_emubd_sync(const struct lfs3_cfg *cfg);
 
 
-/// Additional extended API for driving test features ///
+/// Additional emubd features for testing ///
 
 // Set the current prng state
 void lfs3_emubd_seed(const struct lfs3_cfg *cfg, uint32_t seed);