ipxe-boot-server: allow distinguishing between UEFI and MBR boots

Right now, we hard-code initrd=initrd in the kernel cmdline. This allows booting Linux on UEFI firmwares, but breaks the MBR mode.

Since we cannot have a single kernel cmdline that works for both UEFI and MBR, we may want to be able to detect between the two, so we can add initrd=initrd in the UEFI codepath.