futility/file_type_bios.c: Skip keyblock checks if magic is invalid

The following warning messages appear every time upstream coreboot is
built with Vboot enabled:

```
WARNING: prepare_slot: VBLOCK_A keyblock is invalid.
WARNING: prepare_slot: VBLOCK_B keyblock is invalid.
```

This patch ensures that the warnings are not printed, by skipping checks
if the keyblock magic is invalid. A new keyblock will be generated
without extraneous warnings.

Change-Id: If8e653036b4ca1d10f28bc3bab105b38b152edcd
Signed-off-by: Michał Kopeć <[email protected]>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/5682443
Tested-by: Julius Werner <[email protected]>
Reviewed-by: Yu-Ping Wu <[email protected]>
Commit-Queue: Julius Werner <[email protected]>
1 file changed