MFC r345438,r345842,r346259,r346261: TPM as possible entropy source

r345438:
Allow using TPM as entropy source

TPM has a built-in RNG, with its own entropy source.
The driver was extended to harvest 16 random bytes from TPM every 10 seconds.
A new build option "TPM_HARVEST" was introduced - for now, however, it
is not enabled by default in the GENERIC config.

Reviewed by: markm, delphij
Approved by: secteam

r345842:
Add a cv_wait to the TPM2.0 harvesting function

r346259:
tpm: Prevent session hijack

r346261:
Improve tpm20 style

Submitted by: Kornel Duleba <mindal@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield