Update meta-psa/README.md with eCryptfs passphrase management information

[simonqhughes]

The following provides more information on this commit:

• The psa_trusted_storage_linux system integration stores the top level passphrase in insecure local flash. The passphrase is used to encrypt the FEKEK and hence this state of affairs is inadequate. For a production system, the passphrase must be stored securely e.g. in a Trusted Platform Module or Secure Element. Further platform specific integration is therefore required.
• The README.md file has been updated with guidance information on how the top level passphrase can be stored securely.

[simonqhughes]

@jjohnson-arm Thanks for the commit suggestions. All accepted.