Hi, what is the controller version on HP870 controller? You shouldnt really need to do tftp procedure for each APs.
Try using the DHCP option 43 to specify the HP870 controller ip address in your DHCP scope. Attached is a simplified tool to create option 43 ASCII code for respective controller IP address. Extract and open the "option43tool2.exe" from your command prompt, Once you key in the controller ip address it will generate ASCII character equivalent in two variant one for MSM and another for Unified.
Use the MSM variant of the ASCII character to set in your DHCP scope.
This should help the MSM APs to find the HP870 controllers.