The server becomes unavailable when you use Server Diagnostics. You can press the Option key at startup to start up Server Diagnostics Mac OS X or Server Diagnostics EFI.
Our upcoming UEFI Forum hosted webinar will introduce Arm SystemReady program and show-cases enablement efforts for devices under this program, using open source firmware projects such as TianoCore and U-Boot.
However, if you are running one of the 16 Mac models identified by Duo Labs, there is a chance that your system hasn't received any EFI update at all: iMac 7,1 to 10,1, MacBook 5,1 and 5,2, MacBook Air 2. Apple released EFI updates separately, but since 2015 the EFI update is bundled in the software update Apple has been pushing out to users.
