Increased default lookahead to 512 blocks

Because lookahead is stored efficiently as a bit-vector, this only
requires a ram increase of 48 bytes (2.1% of benchmark), but decreases
the SD benchmark runtime cost by 32 seconds (21.9% of benchmark).

Note this is unimportant on devices with byte-reads such as NOR flash.

mbed_lib.json

@@ -18,7 +18,7 @@
     },
     "lookahead": {
         "macro_name": "MBED_LFS_LOOKAHEAD",
-        "value": 128,
+        "value": 512,
         "help": "Number of blocks to lookahead during block allocation. A larger lookahead reduces the number of passes required to allocate a block. The lookahead buffer requires only 1 bit per block so it can be quite large with little ram impact. Should be a multiple of 32."
     }
   }