build: fix exported variable name CFLAGS_stack_boundary

build: fix exported variable name CFLAGS_stack_boundary

Exporting a variable with a dash doesn't work reliably, they may be
striped from the environment when calling a sub-make or sub-shell.

CFLAGS-stack-boundary start to be removed from env in patch "build:
set ALL_OBJS in main Makefile; move prelink.o to main Makefile" when
running `make "ALL_OBJS=.."` due to the addition of the quote. At
least in my empirical tests.

Fixes: 2740d96efd ("xen/build: have the root Makefile generates the CFLAGS")
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>