x86/boot: check for not allowed sections before linking

x86/boot: check for not allowed sections before linking

Currently check for not allowed sections is performed just after
compilation. However, if compilation succeeds and check fails then
second build will create xen.gz/xen.efi without any visible error.
This happens because %.o: %.c recipe created object file during first
run and make do not execute this recipe during second run. So, look
for not allowed sections before linking. This way check will be
executed every time.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>