modules: littlefs copy the naive crc function from LittleFS

LittleFS requires a `lfs_crc()` function, however the basic crc
function provided by LittleFS gets disabled when a custom configuration
header is used.

To ensure identical crc behavior let Zephyr have a copy of the basic
lfs_crc.

This allows to ensure identical behavior when moving glue code into
Zephyr repo, and allow us to change to an existing crc implementation
provided natively by Zephyr at a later time.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

modules/littlefs/zephyr_lfs_crc.c

@@ -0,0 +1,31 @@
+/*
+ * lfs util functions
+ *
+ * Copyright (c) 2017, Arm Limited. All rights reserved.
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+#include "lfs_util.h"
+
+/* Use the LFS naive CRC implementation until it has been decided which CRC to
+ * use for LittleFS.
+ */
+
+/* Software CRC implementation with small lookup table */
+uint32_t lfs_crc(uint32_t crc, const void *buffer, size_t size)
+{
+	static const uint32_t rtable[16] = {
+		0x00000000, 0x1db71064, 0x3b6e20c8, 0x26d930ac,
+		0x76dc4190, 0x6b6b51f4, 0x4db26158, 0x5005713c,
+		0xedb88320, 0xf00f9344, 0xd6d6a3e8, 0xcb61b38c,
+		0x9b64c2b0, 0x86d3d2d4, 0xa00ae278, 0xbdbdf21c,
+	};
+
+	const uint8_t *data = buffer;
+
+	for (size_t i = 0; i < size; i++) {
+		crc = (crc >> 4) ^ rtable[(crc ^ (data[i] >> 0)) & 0xf];
+		crc = (crc >> 4) ^ rtable[(crc ^ (data[i] >> 4)) & 0xf];
+	}
+
+	return crc;
+}