Update the ima_boot_aggregate apsects of the "README" and "help" files

Add the missing "evmctl ima_boot_aggregate" info to the README.  Update
the "help" to include the new "--pcrs" option.  In addition, replace
the "file" option with "TPM 1.2 BIOS event log".  The new format is:

ima_boot_aggregate [--pcrs hash-algorithm,file] [TPM 1.2 BIOS event log]

Reminder: calculating the TPM PCRs based on the BIOS event log and
comparing them with the TPM PCRs should be done prior to calculating the
possible boot_aggregate value(s).

For TPM 1.2, the TPM 1.2 BIOS event log may be provided as an option
when calculating the ima_boot_aggregate.  For TPM 2.0, "tsseventextend
-sim -if <binary_bios_measurements> -ns -v", may be used to validate
the TPM 2.0 event log.

(Note: some TPM 2.0's export the BIOS event log in the TPM 1.2 format.)

Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

README

@@ -28,6 +28,7 @@ COMMANDS
  import [--rsa] pubkey keyring
  sign [-r] [--imahash | --imasig ] [--portable] [--key key] [--pass password] file
  verify file
+ ima_boot_aggregate [--pcrs hash-algorithm,file] [TPM 1.2 BIOS event log]
  ima_sign [--sigfile] [--key key] [--pass password] file
  ima_verify file
  ima_hash file