[cloud] Add support for AWS IMDSv2

Use an HTTP PUT request to fetch a session token, and pass this token
value as a header when fetching the user-data script.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/cloud/aws.ipxe

@@ -19,6 +19,22 @@ exit
 
 :dhcp_ok
 route ||
-chain -ar http://169.254.169.254/latest/user-data ||
+
+# Fetch session token
+params --method PUT
+param --header X-Aws-Ec2-Metadata-Token-Ttl-Seconds 600
+imgset authtoken http://169.254.169.254/latest/api/token##params ||
+
+# Fetch user-data
+params
+isset ${authtoken} && param --header X-Aws-Ec2-Metadata-Token ${authtoken} ||
+kernel http://169.254.169.254/latest/user-data##params ||
+imgstat ||
+
+# Boot from user-data
+boot -ar ||
+
+# Show diagnostic information on failure
+imgstat ||
 ifstat ||
 exit