utils.sh: use mbedtls gzipped tarball instead of zip file

Compress gzip tarballs (tar.gz) files have better compression ratios
compare to zipped ones as seen below:

    -rw-rw-r-- 1 lsandov1 lsandov1 2809466 sep  9 14:12 mbedtls-2.18.0.tar.gz
    -rw-rw-r-- 1 lsandov1 lsandov1 3428845 sep  9 14:06 mbedtls-2.18.0.zip

Change-Id: I60fb417d78d0ca028878d514e448fead10f277c2
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
1 file changed