Peter Chubb <peterc@gelato.unsw.edu.au> reported that building i386
on a non-i386 platform failed, because gcc could not locate boot.h.
Root cause was the extra include2 directory used when using O=
to specify the output directory.
Added LINUXINCLUDE as a portable way to specify the include/
directory, and changed the two users.
This avoids hardcoding 'include2' in non-kbuild core files.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>