add(duo config):support SPINOR image compilation on IOB board

buildroot-2021.05/configs/milkv-duo-spinor_musl_riscv64_defconfig

@@ -0,0 +1,41 @@
+BR2_riscv=y
+BR2_TOOLCHAIN_EXTERNAL=y
+BR2_TOOLCHAIN_EXTERNAL_CUSTOM_PREFIX="riscv64-unknown-linux-musl"
+BR2_TOOLCHAIN_EXTERNAL_HEADERS_5_10=y
+BR2_TOOLCHAIN_EXTERNAL_CUSTOM_MUSL=y
+BR2_TOOLCHAIN_EXTERNAL_CXX=y
+BR2_TARGET_LDFLAGS="-mcpu=c906fdv -march=rv64imafdcv0p7xthead -mcmodel=medany -mabi=lp64d"
+BR2_TARGET_GENERIC_HOSTNAME="milkv-duo"
+BR2_TARGET_GENERIC_ISSUE="Welcome to Milk-V Duo"
+BR2_TARGET_GENERIC_ROOT_PASSWD="milkv"
+BR2_ROOTFS_OVERLAY="board/milkv/milkv-duo-spinor/overlay"
+BR2_PACKAGE_DUO_PINMUX=y
+BR2_PACKAGE_SPIDEV_TEST=y
+BR2_PACKAGE_STRACE=y
+BR2_PACKAGE_E2FSPROGS=y
+BR2_PACKAGE_E2FSPROGS_RESIZE2FS=y
+BR2_PACKAGE_MTD=y
+BR2_PACKAGE_EVTEST=y
+BR2_PACKAGE_OPENSSL=y
+BR2_PACKAGE_FREETYPE=y
+BR2_PACKAGE_EXPAT=y
+BR2_PACKAGE_JSON_C=y
+BR2_PACKAGE_LIBXSLT=y
+BR2_PACKAGE_LIBNL=y
+BR2_PACKAGE_LIBFFI=y
+BR2_PACKAGE_DHCPCD=y
+BR2_PACKAGE_DNSMASQ=y
+BR2_PACKAGE_DROPBEAR=y
+BR2_PACKAGE_ETHTOOL=y
+BR2_PACKAGE_IPERF3=y
+BR2_PACKAGE_NTP=y
+BR2_PACKAGE_NTP_NTPDATE=y
+BR2_PACKAGE_NTP_NTPTIME=y
+BR2_PACKAGE_HTOP=y
+BR2_PACKAGE_KMOD=y
+BR2_PACKAGE_UTIL_LINUX_UUIDD=y
+BR2_TARGET_ROOTFS_SQUASHFS=y
+BR2_TARGET_ROOTFS_SQUASHFS4_XZ=y
+# BR2_TARGET_ROOTFS_TAR is not set
+BR2_PACKAGE_HOST_E2FSPROGS=y
+BR2_PACKAGE_HOST_GENIMAGE=y