build_package.sh: strip first component from mbedtls archive

Mbedtls tarballs (or zips) are packaged including the
<projects-name>-<version> as root folder, so this one must be stripped
when extracting it for a proper setup.

Change-Id: Iea2b4af0655e93b31bdd5122d65d43b8144570d9
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
diff --git a/utils.sh b/utils.sh
index 56d8933..7c63dc9 100644
--- a/utils.sh
+++ b/utils.sh
@@ -234,14 +234,15 @@
 extract_tarball() {
 	local archive="$1"
 	local target_dir="$2"
+	local extra_params="${3:-}"
 
 	pushd "$target_dir"
 	case $(file --mime-type -b "$archive") in
 		application/gzip)
-				tar -xzf $archive
+				tar -xz $extra_params -f $archive
 				;;
 		application/zip)
-				unzip -q $archive
+				unzip -q $extra_params $archive
 				;;
 	esac
 	popd "$target_dir"