x86, intel_txt: clean up the impact on generic code, unbreak non-x86

Move tboot.h from asm to linux to fix the build errors of intel_txt
patch on non-X86 platforms. Remove the tboot code from generic code
init/main.c and kernel/cpu.c.

Signed-off-by: Shane Wang <shane.wang@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

security/Kconfig

@@ -115,7 +115,7 @@ config SECURITY_ROOTPLUG
 
 config INTEL_TXT
 	bool "Enable Intel(R) Trusted Execution Technology (Intel(R) TXT)"
-	depends on EXPERIMENTAL && X86 && DMAR && ACPI
+	depends on HAVE_INTEL_TXT
 	help
 	  This option enables support for booting the kernel with the
 	  Trusted Boot (tboot) module. This will utilize