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.c

@@ -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.
@@ -1030,7 +1034,7 @@ int lfs3_emubd_sync(const struct lfs3_cfg *cfg) {
 }
 
 
-/// Additional extended API for driving test features ///
+/// Additional emubd features for testing ///
 
 void lfs3_emubd_seed(const struct lfs3_cfg *cfg, uint32_t seed) {
     LFS3_EMUBD_TRACE("lfs3_emubd_seed(%p, 0x%08"PRIx32")",