Performance improvements

- Use larger buffer size when writing uncompressed files
  (was libcurl's default of 16 kb, change it to 128 kb)
- Uncompress the next MB of data, while it is hashing/writing to
  disk in seperate thread.

config.h

@@ -22,6 +22,9 @@
 /* Block size used for writes (currently used when using .zip images only) */
 #define IMAGEWRITER_BLOCKSIZE             1*1024*1024
 
+/* Block size used with uncompressed images */
+#define IMAGEWRITER_UNCOMPRESSED_BLOCKSIZE 128*1024
+
 /* Block size used when reading during verify stage */
 #define IMAGEWRITER_VERIFY_BLOCKSIZE      128*1024