PXE-booting via EFI

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

This question is mostly not NetBSD-specific, but I'm not sure the answers 
I found elswhere are technically correct.

When PXE-booting in legacy BIOS mode, the filename that the DHCP server 
offers in the first step is pxeboot_ia32.bin. When PXE-booting in EFI mode,
that filename should be bootx64.efi.

How can I diffentiate between the two methods in dhcpd.conf?
Common wisdom seems to check option architecture-type = 00:07, 
but there are also fields in option vendor-class-identifier after 
"PXEclient:" that seem to differ.

Probably NetBSD-specific: In the second stage (PXE boot loading the kernel), 
in legacy mode, vendor-class-identifier is "NetBSD:i386:libsa", while in EFI 
mode, it seems to be "NetBSD:amd64:libsa" (note "amd64", not "x86_64"). 
Is this guaranteed behaviour?