Review comments for TPM attestation

Lots of comments and an updated boot test. Generously increase the timeout to eliminate random CI failures.

Test Plan: Boot test works

Bug: T499

X-Origin-Diff: phab/D319
GitOrigin-RevId: cf17fe7c599f670ff8b6f0ac60486f2a04f13a5a
diff --git a/core/pkg/tpm/tpm.go b/core/pkg/tpm/tpm.go
index bb92289..d659d3a 100644
--- a/core/pkg/tpm/tpm.go
+++ b/core/pkg/tpm/tpm.go
@@ -265,7 +265,7 @@
 
 func loadAK() error {
 	var err error
-	// Rationale: The AK is a EK-equivalent key and used only for attestation. Using a non-primary
+	// Rationale: The AK is an EK-equivalent key and used only for attestation. Using a non-primary
 	// key here would require us to store the wrapped version somewhere, which is inconvenient.
 	// This being a primary key in the Endorsement hierarchy means that it can always be recreated
 	// and can never be "destroyed". Under our security model this is of no concern since we identify