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/prepare-sdl.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -e
+DIR=$(dirname ${BASH_SOURCE[0]})
+cd "$DIR"
+. common
+mkdir -p "$PREBUILT_DATA_DIR"
+cd "$PREBUILT_DATA_DIR"
+
+DEP_DIR=SDL2-2.0.20
+
+FILENAME=SDL2-devel-2.0.20-mingw.tar.gz
+SHA256SUM=38094d82a857d6c62352e5c5cdec74948c5b4d25c59cbd298d6d233568976bd1
+
+if [[ -d "$DEP_DIR" ]]
+then
+    echo "$DEP_DIR" found
+    exit 0
+fi
+
+get_file "https://libsdl.org/release/$FILENAME" "$FILENAME" "$SHA256SUM"
+
+mkdir "$DEP_DIR"
+cd "$DEP_DIR"
+
+TAR_PREFIX="$DEP_DIR" # root directory inside the tar has the same name
+tar xf "../$FILENAME" --strip-components=1 \
+    "$TAR_PREFIX"/i686-w64-mingw32/bin/SDL2.dll \
+    "$TAR_PREFIX"/i686-w64-mingw32/include/ \
+    "$TAR_PREFIX"/i686-w64-mingw32/lib/ \
+    "$TAR_PREFIX"/x86_64-w64-mingw32/bin/SDL2.dll \
+    "$TAR_PREFIX"/x86_64-w64-mingw32/include/ \
+    "$TAR_PREFIX"/x86_64-w64-mingw32/lib/ \