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_rambd.h

@@ -29,7 +29,7 @@ struct lfs3_rambd_cfg {
 
 // rambd state
 typedef struct lfs3_rambd {
-    uint8_t *buffer;
+    uint8_t *mem;
     const struct lfs3_rambd_cfg *cfg;
 } lfs3_rambd_t;