fix application on `nanox` devices

[ryankurte]

the pre-release nanox firmware launches but crashes when requesting account keys and during transaction signing, this does not occur in the simulator or when compiling the same application for the nanosplus.

we've tried:

• reducing stack use further which resolved the account-keys crash, getting us to the crash in transaction signing
  (stack use improvements (towards nanox execution) #23)
• tested swapping from armv6 to armv7 which makes computations dramatically more efficient
  (and tested both v6 and v7 ISA applications on the nanosplus, where they work as expected)

[ryankurte]

update: nanox requires SP to never exceed 8k where the nanos only requires SP < 8k when making syscalls. further optimisation of stack frames in #45 has us under this limit, though it is yet to be tested on a nanox.

[ryankurte]

further stack reduction did not resolve the issue on the nanox, however, ledger have a fix in the works that resolves this limitation, so for the moment we're blocked on this.