Improve prebuilt system

This aims to fix two issues with the previous implementation:
 1. the whole content of downloaded archives were extracted, while only
    few files are necessary;
 2. the archives were extracted in the prebuild-deps/ directory as is.

As a consequence of (2), the actual directory name relied on the root
directory of the archive. For adb, this root directory was always
"platform-tools", so when bumping the adb version, the target directory
already existed and the dependency was not upgraded (the old one had to
be removed manually).

Expose common function to download a file and check its checksum, but
let the custom script for each dependency extract only the needed files
and reorganize the content if necessary.

cross_win32.txt

@@ -19,5 +19,5 @@ endian = 'little'
 ffmpeg_avcodec = 'avcodec-58'
 ffmpeg_avformat = 'avformat-58'
 ffmpeg_avutil = 'avutil-56'
-prebuilt_ffmpeg = 'ffmpeg-4.3.1-win32-shared'
+prebuilt_ffmpeg = 'ffmpeg-win32-4.3.1'
 prebuilt_sdl2 = 'SDL2-2.0.20/i686-w64-mingw32'