kbuild: use shorten log for linking u-boot

Move sandbox-specific link rule to arch/sandbox/config.mk.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/sandbox/config.mk b/arch/sandbox/config.mk
index 6142dd4..23b057e 100644
--- a/arch/sandbox/config.mk
+++ b/arch/sandbox/config.mk
@@ -7,3 +7,7 @@
 
 # Support generic board on sandbox
 __HAVE_ARCH_GENERIC_BOARD := y
+
+cmd_u-boot__ = $(CC) -o $@ -T u-boot.lds \
+	-Wl,--start-group $(u-boot-main) -Wl,--end-group \
+	$(PLATFORM_LIBS) -Wl,-Map -Wl,u-boot.map