riscv,fpga_defconfig: point initramfs path to the rootfs repo

arch/riscv/configs/fpga_defconfig

@@ -175,7 +175,7 @@ CONFIG_NET_NS=y
 # CONFIG_SYSFS_DEPRECATED is not set
 # CONFIG_RELAY is not set
 CONFIG_BLK_DEV_INITRD=y
-CONFIG_INITRAMFS_SOURCE="arch/riscv/rootfs/initramfs.txt"
+CONFIG_INITRAMFS_SOURCE="${RISCV_ROOTFS_HOME}/rootfsimg/initramfs.txt"
 CONFIG_INITRAMFS_ROOT_UID=0
 CONFIG_INITRAMFS_ROOT_GID=0
 CONFIG_RD_GZIP=y