eve-core: enable EROFS + DM_VERITY for split-rootfs

[eriknordmark]

The split-rootfs feature delivers EVE's non-critical services in a separate
Extension image — an EROFS filesystem, dm-verity protected — that
extsloader opens with veritysetup and mounts read-only at /persist/exts.
The stock eve-core_defconfig enables neither EROFS nor dm-verity, so on a
stock-kernel build the Extension cannot mount; an OTA to a split image then
fails the extension-readiness check and the device rolls back to the previous
image.

This enables the three options the feature needs in arch/x86/configs/eve-core_defconfig:

• CONFIG_EROFS_FS and CONFIG_EROFS_FS_ZIP — the Extension is built
  lz4hc-compressed (mkfs.erofs -zlz4hc), so the ZIP/decompression support is
  required to mount it.
• CONFIG_DM_VERITY — the device-mapper base (CONFIG_MD, CONFIG_BLK_DEV_DM)
  and SHA-256 were already present.

Validated end-to-end on amd64/QEMU: with this kernel, extsloader reaches
ready, the verity device opens and the EROFS extension mounts, and an OTA from
an old (300 MB-partition) image to the split/universal image passes the
extension-readiness gate instead of rolling back.

amd64 (eve-core_defconfig) only; the arm64 equivalent is a follow-up.
Opening as draft alongside the in-progress split-rootfs EVE work.

🤖 Generated with Claude Code

[rucoder]

@eriknordmark we need to enable
+CONFIG_EROFS_FS=y
+CONFIG_EROFS_FS_ZIP_LZMA=y
+CONFIG_EROFS_FS_ZIP_ZSTD=y
+CONFIG_EROFS_FS_PCPU_KTHREAD=y

erofs differ may use fater LZMA compression that is not too heavy on CPU

[eriknordmark]

CONFIG_EROFS_FS=y is already in the diff, together with CONFIG_EROFS_FS_ZIP=y. Adding the other three is fine — they all just hang off EROFS_FS_ZIP, so nothing else has to change:

CONFIG_EROFS_FS_ZIP_LZMA=y
CONFIG_EROFS_FS_ZIP_ZSTD=y
CONFIG_EROFS_FS_PCPU_KTHREAD=y

One thing to decide with them: EROFS_FS_PCPU_KTHREAD_HIPRI is default y once PCPU_KTHREAD is on, so enabling it also puts the decompression workers at high priority unless we set # CONFIG_EROFS_FS_PCPU_KTHREAD_HIPRI is not set explicitly. On a device that is also running app workloads I would rather not have them there by default — do you want HIPRI on or off?