Added conversion to/from little-endian on disk

Required to support big-endian processors, with the most notable being
the PowerPC architecture.

On little-endian architectures, these conversions can be optimized out
and have no code impact.

Initial patch provided by gmouchard

lfs_util.h

@@ -49,6 +49,23 @@ static inline int lfs_scmp(uint32_t a, uint32_t b) {
     return (int)(unsigned)(a - b);
 }
 
+static inline uint32_t lfs_fromle32(uint32_t a) {
+#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+    return a;
+#elif defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+    return __builtin_bswap32(a);
+#else
+    return (((uint8_t*)&a)[0] <<  0) |
+           (((uint8_t*)&a)[1] <<  8) |
+           (((uint8_t*)&a)[2] << 16) |
+           (((uint8_t*)&a)[3] << 24);
+#endif
+}
+
+static inline uint32_t lfs_tole32(uint32_t a) {
+    return lfs_fromle32(a);
+}
+
 // CRC-32 with polynomial = 0x04c11db7
 void lfs_crc(uint32_t *crc, const void *buffer, size_t size);