Adopted erase_size/erase_count config in test block-devices/runners

In separating the configuration of littlefs from the physical geometry
of the underlying device, we can no longer rely solely on lfs_config to
contain all of the information necessary for the simulated block devices
we use for testing.

This adds a new lfs_*bd_config struct for each of the block devices, and
new erase_size/erase_count fields. The erase_* name was chosen since
these reflect the (simulated) physical erase size and count of
erase-sized blocks, unlike the block_* variants which represent logical
block sizes used for littlefs's bookkeeping.

It may be worth adopting erase_size/erase_count in littlefs's config at
some point in the future, but at the moment doesn't seem necessary.

Changing the lfs_bd_config structs to be required is probably a good
idea anyways, as it moves us more towards separating the bds from
littlefs. Though we can't quite get rid of the lfs_config parameter
because of the block-device API in lfs_config. Eventually it would be
nice to get rid of it, but that would require API breakage.

bd/lfs_rambd.h

@@ -26,8 +26,20 @@ extern "C"
 #endif
 #endif
 
-// rambd config (optional)
+// rambd config
 struct lfs_rambd_config {
+    // Minimum size of a read operation in bytes.
+    lfs_size_t read_size;
+
+    // Minimum size of a program operation in bytes.
+    lfs_size_t prog_size;
+
+    // Size of an erase operation in bytes.
+    lfs_size_t erase_size;
+
+    // Number of erase blocks on the device.
+    lfs_size_t erase_count;
+
     // Optional statically allocated buffer for the block device.
     void *buffer;
 };
@@ -39,9 +51,8 @@ typedef struct lfs_rambd {
 } lfs_rambd_t;
 
 
-// Create a RAM block device using the geometry in lfs_config
-int lfs_rambd_create(const struct lfs_config *cfg);
-int lfs_rambd_createcfg(const struct lfs_config *cfg,
+// Create a RAM block device
+int lfs_rambd_create(const struct lfs_config *cfg,
         const struct lfs_rambd_config *bdcfg);
 
 // Clean up memory associated with block device