selftests/liveupdate: add end to end test infrastructure and scripts

Add the end to end testing infrastructure required to verify the
liveupdate feature. This includes a custom init process, a test
orchestration script, and a batch runner.

The framework consists of:

init.c:
A lightweight init process that manages the kexec lifecycle.
It mounts necessary filesystems, determines the current execution
stage (1 or 2) via the kernel command line, and handles the
kexec_file_load() sequence to transition between kernels.

vmtest.sh:
The primary KTAP-compliant test driver. It handles:
- Kernel configuration merging and building.
- Cross-compilation detection for x86_64 and arm64.
- Generation of the initrd containing the test binary and init.
- QEMU execution with automatic accelerator detection (KVM, HVF,
 or TCG).

run-vmtests.sh:
A wrapper that runs vmtest.sh for each LUO test across supported
architectures, providing a summary of pass/fail/skip results.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Co-developed-by: Jordan Richards <jordanrichards@google.com>
Signed-off-by: Jordan Richards <jordanrichards@google.com>
Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://patch.msgid.link/20260626-luo-vmtest-v0-v4-1-e7d3111cd5b3@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

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