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/script/build_package.sh b/script/build_package.sh
index 829b3ec..da75a87 100755
--- a/script/build_package.sh
+++ b/script/build_package.sh
@@ -442,7 +442,7 @@
 
 			url="$mbedtls_archive" saveas="$mbedtls_ar" fetch_file
 			mkdir "$mbedtls_dir"
-			extract_tarball $mbedtls_ar $mbedtls_dir
+			extract_tarball $mbedtls_ar $mbedtls_dir --strip-components=1
 		fi
 
 		emit_env "MBEDTLS_DIR" "$mbedtls_dir"