dasharo: add Dasharo Tools Suite (Nightly) iPXE menu entry

[Houmgaor]

This PR adds a new "Dasharo Tools Suite (Nightly)" entry to the Dasharo iPXE network boot menu, which chains to https://boot.dasharo.com/dts/nightly.ipxe (nightly DTS builds from the develop branch).

The new entry is placed directly after the existing "Dasharo Tools Suite" entry, as requested in the issue. The handler follows the same pattern as the stable DTS entry (dhcp || chain <url> || goto MENU).

Verification

Tested on QEMU Q35 with Dasharo coreboot firmware (v0.2.1-rc1). See screenshots.

To reproduce:

1. Clone coreboot repo and apply the iPXE change
2. Set up swtpm + QEMU with a NIC.
3. In UEFI Setup, enable network boot
4. After reboot, go to One Time Boot → iPXE Network Boot

Before (pre-built v0.2.1-rc1 ROM — 4 entries, no Nightly)

After (custom ROM with modified dasharo.ipxe — 5 entries with Nightly)

Checklist

• Menu entry appears in correct position (after DTS stable, before netboot.xyz)
• Menu label matches expected format: "Dasharo Tools Suite (Nightly)"
• Handler chains to correct URL: https://boot.dasharo.com/dts/nightly.ipxe
• nightly.ipxe endpoint is live and returns a valid iPXE script
• Existing menu entries and behavior are unchanged
• Autoboot default + 3s timeout still works as before

Open items

• OSFV tests: The network-boot.robot tests in open-source-firmware-validation will need updating to expect 5 menu entries instead of 4.
• nightly.ipxe availability: Confirmed the endpoint is live (HTTP 200, serves a valid iPXE script referencing nightly/bzImage and nightly/dts-base-image.cpio.gz). Per the issue, availability of functional nightly builds should be confirmed with @DaniilKl.

Resolves: Dasharo/dasharo-issues#1041