Also reduces the IRAM footprint of the fatal exception handler, as only
the prelude (which disables interrupts & enables the flash mapping) is
in IRAM now.
Closes#54, relevant to #133.
mbedTLS version 2.1.0 (current stable)
Has some known issues/hacks:
* Entropy source not hooked in at all
* Linker script has a messy hack in it to store some (not all) data in
irom