mbedtls-amis: get debug message

export PACKER_LOG=1 to get some more detail debug message while building
the AMI.

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I419e136571906c712b424ee3708dc5b1c3fc09e3
diff --git a/mbedtls-amis/builders.sh b/mbedtls-amis/builders.sh
index 44816d0..114347f 100755
--- a/mbedtls-amis/builders.sh
+++ b/mbedtls-amis/builders.sh
@@ -1,5 +1,7 @@
 #!/bin/bash
 
+export PACKER_LOG=1
+
 AMI=""
 [ ${BUILD_UBUNTU_AMI} == "true" ] && AMI="${AMI} ubuntu"
 [ ${BUILD_FREEBSD_AMI} == "true" ] && AMI="${AMI} freebsd"