signing: add condition for flexor

Looking at `ImageType` enum in the proto def, we use a type
`IMAGE_TYPE_FLEXOR_KERNEL` for uefi signing. The old signing
instructions would translate that to uefi, but new signing just strips
the prefix and passes through `flexor_kernel`. Instead of having to do
mapping, let's just make all the code consistent and easier to reason
about.

BUG=b:372702794
TEST=None

Change-Id: I90c0bfc9afe0daea23a3f4e2fc0cac6f88779fe5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/6165423
Reviewed-by: George Engelbrecht <[email protected]>
Commit-Queue: ChromeOS Auto Retry <[email protected]>
Reviewed-by: Madeleine Hardt <[email protected]>
Tested-by: Benjamin Shai <[email protected]>
1 file changed