Make asm-x86/bootparam.h includable from userspace.

To actually write a bootloader (or, say, the lguest launcher)
currently requires duplication of these structures.  Making them
includable from userspace is much nicer.

We merge the common userspace-required definitions of e820_32/64.h
into e820.h for export.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>