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/unmatched/Makefile b/board/sifive/unmatched/Makefile
index e00b330..1345330 100644
--- a/board/sifive/unmatched/Makefile
+++ b/board/sifive/unmatched/Makefile
@@ -2,9 +2,10 @@
 #
 # Copyright (c) 2020-2021 SiFive, Inc
 
-obj-y   += unmatched.o
 obj-$(CONFIG_ID_EEPROM) += hifive-platform-i2c-eeprom.o
 
 ifdef CONFIG_SPL_BUILD
 obj-y += spl.o
+else
+obj-y += unmatched.o
 endif