x86: don't build with EFI support in shim-exclusive mode

There's no need for xen.efi at all, and there's also no need for EFI
support in xen.gz since the shim runs in PVH mode, i.e. without any
firmware (and hence by implication also without EFI one).

The slightly odd looking use of $(space) is to ensure the new ifneq()
evaluates consistently between "build" and "install" invocations of
make.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/Makefile

@@ -80,7 +80,9 @@ x86_emulate.o: x86_emulate/x86_emulate.c x86_emulate/x86_emulate.h
 
 efi-y := $(shell if [ ! -r $(BASEDIR)/include/xen/compile.h -o \
                       -O $(BASEDIR)/include/xen/compile.h ]; then \
-                         echo '$(TARGET).efi'; fi)
+                         echo '$(TARGET).efi'; fi) \
+         $(space)
+efi-$(CONFIG_PV_SHIM_EXCLUSIVE) :=
 
 ifneq ($(build_id_linker),)
 notes_phdrs = --notes
@@ -113,11 +115,13 @@ $(TARGET): $(TARGET)-syms $(efi-y) boot/mkelf32
 		{ echo "No Multiboot2 header found" >&2; false; }
 	mv $(TMP) $(TARGET)
 
+ifneq ($(efi-y),)
 # Check if the compiler supports the MS ABI.
 export XEN_BUILD_EFI := $(shell $(CC) $(XEN_CFLAGS) -c efi/check.c -o efi/check.o 2>/dev/null && echo y)
 # Check if the linker supports PE.
 XEN_BUILD_PE := $(if $(XEN_BUILD_EFI),$(shell $(LD) -mi386pep --subsystem=10 -o efi/check.efi efi/check.o 2>/dev/null && echo y))
 CFLAGS-$(XEN_BUILD_EFI) += -DXEN_BUILD_EFI
+endif
 
 ALL_OBJS := $(BASEDIR)/arch/x86/boot/built_in.o $(BASEDIR)/arch/x86/efi/built_in.o $(ALL_OBJS)
 EFI_OBJS-$(XEN_BUILD_EFI) := efi/relocs-dummy.o