sandbox: Support building an SPL image

When building an SPL image, override the link flags so that it uses the
system libraries. This is similar to the way the non-SPL image is built.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/config.mk

@@ -20,4 +20,9 @@ cmd_u-boot__ = $(CC) -o $@ -Wl,-T u-boot.lds \
 	-Wl,--start-group $(u-boot-main) -Wl,--end-group \
 	$(PLATFORM_LIBS) -Wl,-Map -Wl,u-boot.map
 
+cmd_u-boot-spl = (cd $(obj) && $(CC) -o $(SPL_BIN) -Wl,-T u-boot-spl.lds \
+	-Wl,--start-group $(patsubst $(obj)/%,%,$(u-boot-spl-main)) \
+	$(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) -Wl,--end-group \
+	$(PLATFORM_LIBS) -Wl,-Map -Wl,u-boot-spl.map -Wl,--gc-sections)
+
 CONFIG_ARCH_DEVICE_TREE := sandbox