CONFIG: btrfs requires sha256

[Ferry Toth]

In v2021.01-rc2 enabling CONFIG_FS_BTRFS=y produces:

x86_64-poky-linux-ld.bfd: fs/built-in.o: in function `hash_sha256':
     u-boot/1_2021.01-rc2-r0/git/fs/btrfs/crypto/hash.c:25: undefined 
reference to `sha256_starts'
x86_64-poky-linux-ld.bfd: 
u-boot/1_2021.01-rc2-r0/git/fs/btrfs/crypto/hash.c:26: undefined 
reference to `sha256_update'
x86_64-poky-linux-ld.bfd: 
u-boot/1_2021.01-rc2-r0/git/fs/btrfs/crypto/hash.c:27: undefined 
reference to `sha256_finish'

Enabling CONFIG_SHA256=y resolves this, however it is not a dependency.