warp7: panic: hab: Call into BootROM failsafe on panic path

This patch adds a callback into the BootROM's provided High Assurance Boot
(HAB) failsafe function when panicking i.e. the call is done without making
use of stack.

The HAB failsafe function allows a piece of software to call into the
BootROM and place the processor into failsafe mode.

Failsafe mode is a special mode which presents a serial download protocol
interface over UART or USB at the time of writing.

If the board has been set into secure mode, then only a signed binary can
be used to recover the board.

Thus failsafe gives a putatively secure method of performing a secure
recovery over UART or USB.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>