failed to build u-boot mt7621_rfb_defconfig

[Xiaobo Liu]

I checkout u-boot tag: v2023.10

https://github.com/u-boot/u-boot/tree/v2023.10

run build:

```shell
export
STAGING_DIR=/home/foo/openwrt-toolchain-22.03.4-ramips-mt7621_gcc-11.2.0_musl.Linux-x86_64/

make mt7621_rfb_defconfig
make
CROSS_COMPILE=/home/foo/openwrt-toolchain-22.03.4-ramips-mt7621_gcc-11.2.0_musl.Linux-x86_64/toolchain-mipsel_24kc_gcc-11.2.0_musl/bin/mipsel-openwrt-linux-musl-
```

build failed:

```shell
  CC      spl/common/spl/spl.o
  AR      spl/common/spl/built-in.o
  LD      spl/u-boot-spl
  OBJCOPY spl/u-boot-spl-nodtb.bin
  COPY    spl/u-boot-spl.bin
  SYM     spl/u-boot-spl.sym
  MKIMAGE u-boot.img
  MKIMAGE u-boot-dtb.img
  LZMA    u-boot.bin.lzma
  MKIMAGE u-boot-lzma.img
  BINMAN  .binman_stamp
Image 'u-boot-spl-ddr' is missing external blobs and is non-functional:
stage_bin

/binman/u-boot-spl-ddr/stage_bin (mt7621_stage_sram.bin):
   Missing blob

Some images are invalid
make: *** [Makefile:1115: .binman_stamp] Error 103
```

Is there a step I'm missing that's causing this?