Replaced emubd with rambd and filebd

The idea behind emubd (file per block), was neat, but doesn't add much
value over a block device that just operates on a single linear file
(other than adding a significant amount of overhead). Initially it
helped with debugging, but when the metadata format became more complex
in v2, most debugging ends up going through the debug.py script anyways.

Aside from being simpler, moving to filebd means it is also possible to
mount disk images directly.

Also introduced rambd, which keeps the disk contents in RAM. This is
very useful for testing where it increases the speed _significantly_.
- test_dirs w/ filebd - 0m7.170s
- test_dirs w/ rambd  - 0m0.966s

These follow the emubd model of using the lfs_config for geometry. I'm
not convinced this is the best approach, but it gets the job done.

I've also added lfs_ramdb_createcfg to add additional config similar to
lfs_file_opencfg. This is useful for specifying erase_value, which tells
the block device to simulate erases similar to flash devices. Note that
the default (-1) meets the minimum block device requirements and is the
most performant.

rambd/lfs_rambd.h

@@ -0,0 +1,67 @@
+/*
+ * Block device emulated in RAM
+ *
+ * Copyright (c) 2017, Arm Limited. All rights reserved.
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+#ifndef LFS_RAMBD_H
+#define LFS_RAMBD_H
+
+#include "lfs.h"
+#include "lfs_util.h"
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+// rambd config (optional)
+struct lfs_rambd_config {
+    // 8-bit erase value to simulate erasing with. -1 indicates no erase
+    // occurs, which is still a valid block device
+    int32_t erase_value;
+
+    // Optional statically allocated buffer for the block device.
+    void *buffer;
+};
+
+// rambd state
+typedef struct lfs_rambd {
+    uint8_t *buffer;
+    const struct lfs_rambd_config *cfg;
+} 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,
+        const struct lfs_rambd_config *ramcfg);
+
+// Clean up memory associated with block device
+void lfs_rambd_destroy(const struct lfs_config *cfg);
+
+// Read a block
+int lfs_rambd_read(const struct lfs_config *cfg, lfs_block_t block,
+        lfs_off_t off, void *buffer, lfs_size_t size);
+
+// Program a block
+//
+// The block must have previously been erased.
+int lfs_rambd_prog(const struct lfs_config *cfg, lfs_block_t block,
+        lfs_off_t off, const void *buffer, lfs_size_t size);
+
+// Erase a block
+//
+// A block must be erased before being programmed. The
+// state of an erased block is undefined.
+int lfs_rambd_erase(const struct lfs_config *cfg, lfs_block_t block);
+
+// Sync the block device
+int lfs_rambd_sync(const struct lfs_config *cfg);
+
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif