mbedtls-amis.yaml: Dump packer_config.json to the job log

Dump packer_config.json so that we would know the detail of this AMI
from the job log

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I4e2bc00301a8cbbaf362f07639565650a3990582
diff --git a/mbedtls-amis/builders.sh b/mbedtls-amis/builders.sh
index be49110..95747b2 100644
--- a/mbedtls-amis/builders.sh
+++ b/mbedtls-amis/builders.sh
@@ -4,5 +4,8 @@
 git clone https://git.trustedfirmware.org/next/ci/mbedtls-amis.git
 cd mbedtls-amis
 
+# Dump packer_config.json to job log
+cat packer_config.json
+
 packer build packer_config.json