Generate UKI file for DTS and iPXE

[m-iwanicki]

@macpijan What do you think about this PR?
We could use it as a first step to creating SB capable image (built from kas.yml not separate one).
To enable SB when booting via USB we would need couple more changes (add at least shim)
From my testing grub.cfg file is used only on EFI platform. When booting on legacy BIOS, old config is used.
This PR creates 2 files:

• dts.efi - UKI containing linux stub and kernel. It's in target image under /boot/EFI/DTS/dts.efi and booted via chainloader command
• ipxe_dts.efi - UKI containing linux stub, kernel and rootfs (as initrd). It can be booted via iPXE (e.g. by chain command).
  It can be signed and deployed to boot.dasharo.com via our CI. It would allow booting DTS via iPXE with secure boot enabled (after couple changes in dts.ipxe script).

[m-iwanicki]

Related to: Dasharo/dasharo-issues#1218 (comment)