board: sifive: compile stuff only related to SPL in SPL build

As (3581811dc26f "riscv: sifive/fu540: Move SPL related functions to spl.c"),
we put the SPL stuff in spl.c, we don't need to compile unleashed.c and
unmatched.c in SPL build.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/board/sifive/unleashed/Makefile b/board/sifive/unleashed/Makefile
index 5821679..98e9111 100644
--- a/board/sifive/unleashed/Makefile
+++ b/board/sifive/unleashed/Makefile
@@ -2,8 +2,8 @@
 #
 # Copyright (c) 2019 Western Digital Corporation or its affiliates.
 
-obj-y	+= unleashed.o
-
 ifdef CONFIG_SPL_BUILD
 obj-y += spl.o
+else
+obj-y += unleashed.o
 endif