Add null terminator at the end of entry data

[zundamo-ti]

Fix: add UCS-2 null terminator to --unicode optional data

Fixes #229

When --unicode is used, the optional data in the EFI load option is not null-terminated. This causes shim's split_load_options() to fail to extract the second-stage loader path, as it requires a space or null delimiter to identify the end of the path string.

Changes

In make_var():

• Allocate 2 extra bytes when --unicode is set
• Write UCS-2 null terminator at the end of the load option data before calling efi_set_variable()

Testing

Tested on Azure CVM (SEV-SNP) with Ubuntu 24.04 and shim + UKI boot chain.

Before fix:

• efibootmgr --create --unicode produces optional data without null terminator
• Boot entry fails, falls back to next entry in BootOrder

After fix:

• Optional data correctly ends with 00 00
• Boot entry matches byte-for-byte with firmware-created entries
• Boot succeeds from the new entry