Expand volume size from 20GB to 30GB.

While running the mbedtls-nightly-tests, it failed due to no space left
on the disk [1].

[1]: https://ci.staging.trustedfirmware.org/job/arthur-mbed-tls-nightly-tests/15/console

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I1cfaaec422fa7e0ba67fc77974c6615b2b7aa1ac
diff --git a/packer_config.json b/packer_config.json
index abfd697..43c09ba 100644
--- a/packer_config.json
+++ b/packer_config.json
@@ -32,7 +32,7 @@
         {
           "device_name": "/dev/sda1",
           "volume_type": "gp2",
-          "volume_size": "20",
+          "volume_size": "30",
           "delete_on_termination": true
         }
       ]