(no major updates with WS2019)

Only Hyper-V Server (free version) is documented to supported boot from USB/Flash storage. Microsoft stated already with 2008R2: Although Microsoft Hyper-V Server 2008 R2 is built with components of Windows Server 2008 R2, some changes have been made specifically to Microsoft Hyper-V Server 2008 R2 in order to support boot from USB.
The documentation for setting up Hyper-V Server 2012 for boot from USB are here: http://technet.microsoft.com/en-us/library/jj733589.aspx

Nano Server doesnt support the USB boot. Only SD Card boot is supported.