riscv: sifive: Rename fu540 board to unleashed

In preparation to add SiFive Unmatched board support, let's rename
the existing fu540 board to unleashed.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 55eaee2..e8494c2 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -17,8 +17,8 @@
 config TARGET_QEMU_VIRT
 	bool "Support QEMU Virt Board"
 
-config TARGET_SIFIVE_FU540
-	bool "Support SiFive FU540 Board"
+config TARGET_SIFIVE_UNLEASHED
+	bool "Support SiFive Unleashed Board"
 
 config TARGET_SIPEED_MAIX
 	bool "Support Sipeed Maix Board"
@@ -55,7 +55,7 @@
 source "board/AndesTech/ax25-ae350/Kconfig"
 source "board/emulation/qemu-riscv/Kconfig"
 source "board/microchip/mpfs_icicle/Kconfig"
-source "board/sifive/fu540/Kconfig"
+source "board/sifive/unleashed/Kconfig"
 source "board/sipeed/maix/Kconfig"
 
 # platform-specific options below