x86: headers cleanup - boot.h

Impact: cleanup

'make headers_check' warn us about leaking of kernel private
(mostly compile time vars) data to userspace in headers. Fix it.

Neither BOOT_HEAP_SIZE, BOOT_STACK_SIZE refs
was found by searching thru net (ie in user-space area)
so fence this all by __KERNEL__ guard.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>