Since SRAM has some data remanence even without power it seems good
hygien to clear all RAM when starting the device so as not to leak
potential sensitive data between device apps.
Instead of allocating the blake2s_ctx in the blake2s() function we
pass it as a pointer as an argument to be able to better control where
the variable is in memory.
In particular, order of LOAD_USS and LOAD_APP_SIZE is not required, but
the need to send both is documented. This is followed up with adjustment
in the host programs' Go code, to try to reinforce this. LoadApp() will
take the secretPhrase parameter (to be hashed as USS), and loadUSS()
will be unexported.
Correct CMD/RSP lengths in pseudo-code.
