mbedtls-amis: add support for custom AMI

This change adds in support for building the
custom AMI

Signed-off-by: Kelley Spoon <kelley.spoon@linaro.org>
Change-Id: Ie23a178324bd79340cd1d68a327d49b90e2ee6be
diff --git a/mbedtls-amis/builders.sh b/mbedtls-amis/builders.sh
index 2dcd4e7..44816d0 100755
--- a/mbedtls-amis/builders.sh
+++ b/mbedtls-amis/builders.sh
@@ -3,6 +3,7 @@
 AMI=""
 [ ${BUILD_UBUNTU_AMI} == "true" ] && AMI="${AMI} ubuntu"
 [ ${BUILD_FREEBSD_AMI} == "true" ] && AMI="${AMI} freebsd"
+[ ${BUILD_CUSTOM_AMI} == "true" ] && AMI="${AMI} custom"
 
 [ -z "${AMI}" ] && echo "Build Nothing.." && exit
 
@@ -18,4 +19,3 @@
     cat packer_${os}.json
     packer build packer_${os}.json
 done
-