Android, FFmpeg: Allow specifying target output in FFpeg install script

The provisioning script install-ffmpeg-android.sh can be useful to run
locally to make sure we build FFmpeg with the same configuration as we
do in CI.

This patch introduce an optional parameter to this provisioning script
that allows us to specify where to install the compiled binaries.

Change-Id: I7ea30bffa4753ac1fab707ed2876ddcbbca06175
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
(cherry picked from commit 07775b2ad2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

coin/provisioning/common/unix/install-ffmpeg-android.sh

@@ -8,6 +8,8 @@ set -ex
 source "${BASH_SOURCE%/*}/../unix/ffmpeg-installation-utils.sh"
 
 os="$1"
+# Optional parameter to set output installation directory. Useful for local builds.
+target_install_dir_param="$2"
 build_type=$(get_ffmpeg_build_type)
 ffmpeg_source_dir=$(download_ffmpeg)
 
@@ -109,5 +111,10 @@ else
     exit 1
 fi
 
+# If parameter is set, use it as the target output directory.
+if [ ! -z $target_install_dir_param ]; then
+  target_dir=$target_install_dir_param
+fi
+
 build_ffmpeg_android "$target_arch" "$target_dir"
 set_ffmpeg_dir_env_var "$envvar" "$target_dir"