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.

prebuilt-deps/.gitignore

@@ -1,4 +1 @@
-*
-!/.gitignore
-!/Makefile
-!/prepare-dep
+/data