UEFI and BIOS

[NiKiZe]

You need different binaries for the different platforms.
The only way to do this is for the DHCP server to detect the arch flag and send the correct binary from there.
As long as you have a DHCP server that can send different binaries for the arch option, then you can have the same embedded script in both binaries that chains to a next stage script over http or tftp which has your menu.

Which DHCP server are you using?

One other note since you mention booting ISOs - don't - that approach is a dead end, make sure you understand why ISOs don't work as boot source directly before spending to much time on it.