x86/efi: Implement arch_ima_efi_boot_mode() in source file

The x86 implementation of arch_ima_efi_boot_mode() uses the global
boot_param state. Move it into a source file to clean up the header.
Avoid potential rebuilds of unrelated source files if boot_params
changes.

Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240112095000.8952-4-tzimmermann@suse.de