kho: add Kconfig option to enable KHO by default

Currently, Kexec Handover must be explicitly enabled via the kernel
command line parameter `kho=on`.

For workloads that rely on KHO as a foundational requirement (such as the
upcoming Live Update Orchestrator), requiring an explicit boot parameter
adds redundant configuration steps.

Introduce CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT. When selected, KHO
defaults to enabled. This is equivalent to passing kho=on at boot. The
behavior can still be disabled at runtime by passing kho=off.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Pasha Tatashin <[e-mail]>
Reviewed-by: Mike Rapoport (Microsoft) <[e-mail]>
Reviewed-by: Pratyush Yadav <[e-mail]>
Cc: Alexander Graf <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Coiby Xu <[e-mail]>
Cc: Dave Vasilevsky <[e-mail]>
Cc: Eric Biggers <[e-mail]>
Cc: Kees Cook <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

kernel/liveupdate/Kconfig

···
 	  Also, enables inspecting the KHO fdt trees with the debugfs binary
 	  blobs.
 
+config KEXEC_HANDOVER_ENABLE_DEFAULT
+	bool "Enable kexec handover by default"
+	depends on KEXEC_HANDOVER
+	help
+	  Enable Kexec Handover by default. This avoids the need to
+	  explicitly pass 'kho=on' on the kernel command line.
+
+	  This is useful for systems where KHO is a prerequisite for other
+	  features, such as Live Update, ensuring the mechanism is always
+	  active.
+
+	  The default behavior can still be overridden at boot time by
+	  passing 'kho=off'.
+
 endmenu

kernel/liveupdate/kexec_handover.c

···
 
 static_assert(sizeof(union kho_page_info) == sizeof(((struct page *)0)->private));
 
-static bool kho_enable __ro_after_init;
+static bool kho_enable __ro_after_init = IS_ENABLED(CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT);
 
 bool kho_is_enabled(void)
 {