x86/cfi: Add boot time hash randomization

In order to avoid known hashes (from knowing the boot image),
randomize the CFI hashes with a per-boot random seed.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221027092842.765195516@infradead.org