U-Boot / SPL: undefined reference 'file_fat_write'

[An Schall]

Dear all,

I am trying to write a file to SD card in spl_mmc.c. During compilation
gcc moans: undefined reference 'file_fat_write'. I don't understand why.
Directly after the (original) code in mmc_load_image_fat():

err = file_fat_read(payloadname,
                (u8 *)spl_image.load_addr, 0);

I insert the following code:

file_fat_write("foo", "foo", 3);

Why doesn't gcc know this function as it is also declared in
include/fat.h just as file_fat_read() is?

Did someone manage to write to fat out of the SPL?

Best,
André