Increase the storage from 30GB to 50GB

When running some jobs, it failed due to no enough space [1]

[1]: https://ci.trustedfirmware.org/blue/organizations/jenkins/mbed-tls-pr-head/detail/PR-5608-head/1/pipeline/395

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I5afa4079b210e5ab59e4bc25bad5e8640f9cd68c
diff --git a/packer_config.json b/packer_config.json
index 43c09ba..331c6b8 100644
--- a/packer_config.json
+++ b/packer_config.json
@@ -32,7 +32,7 @@
         {
           "device_name": "/dev/sda1",
           "volume_type": "gp2",
-          "volume_size": "30",
+          "volume_size": "50",
           "delete_on_termination": true
         }
       ]