bitcoind: use correct endianness for block hashes.

Like txids, we need to reverse them.  We didn't, but then we only used them
to pass to/from bitcoind.  We're about to get them from the block header,
so we need to fix that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/block.c

@@ -36,3 +36,15 @@ struct bitcoin_block *bitcoin_block_from_hex(const tal_t *ctx,
 	tal_free(linear_tx);
 	return b;
 }
+
+bool bitcoin_blkid_from_hex(const char *hexstr, size_t hexstr_len,
+			    struct sha256_double *blockid)
+{
+	return bitcoin_txid_from_hex(hexstr, hexstr_len, blockid);
+}
+
+bool bitcoin_blkid_to_hex(const struct sha256_double *blockid,
+			  char *hexstr, size_t hexstr_len)
+{
+	return bitcoin_txid_to_hex(blockid, hexstr, hexstr_len);
+}