Compile error with SPL_FIT_FULL_CHECK and SPL_LOAD_FIT_FULL enabled

[Johann Neuhauser <jneuhauser@dh-electronics.com>]

Dear developers and Simon,

we wanna run secure boot with U-Boot's SPL_FIT_SIGNATURE and FIT_SIGNATURE on our STM32MP1 boards and discovered the CVE-2021-27097.
To mitigate this vulnerability we wanna enable SPL_LOAD_FIT_FULL and SPL_FIT_FULL_CHECK.
If I compile any U-Boot SPL with the mentioned config symbols after commit 6f3c2d8a, it fails always with the following error message:

Used defconfig: stm32mp15_dhcom_basic_defconfig (+ mentioned configs enabled)
```
...
  LD      spl/lib/built-in.o
  LD      spl/u-boot-spl
/usr/bin/arm-linux-gnueabihf-ld.bfd: common/built-in.o: in function `fit_check_format':
/mnt/work/dev/u-boot/common/image-fit.c:1591: undefined reference to `fdt_check_full'
make[1]: *** [scripts/Makefile.spl:432: spl/u-boot-spl] Error 1
make: *** [Makefile:1941: spl/u-boot-spl] Error 2
```
After diging around to find the cause, we're out of ideas.
Does anyone have a clue why the needed function is not compiled in libfdt for the spl build?

Many thanks in advance.

Best regards,

Johann Neuhauser

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