tpm2: add tpm2_get_legacy_template() and tpm2_get_srk_template()

tpm2: add tpm2_get_legacy_template() and tpm2_get_srk_template()

Add functions to get either the 'legacy' or standard SRK template, for RSA or
ECC. The 'legacy' templates are those used with earlier code, where a transient
key was created to use for tpm sealing; the standard SRK is the persistent
shared key as defined in TCG guidance docs.

This also replaces tpm2_get_primary_template() with the new functions; that
function's use of flags is confusing and unnecessary.