arm: imx: hab: Define rvt_failsafe()

arm: imx: hab: Define rvt_failsafe()

The hab_rvt_failsafe() callback according to the HABv4 documentation:

"This function provides a safe path when image authentication has failed
and all possible boot paths have been exhausted. It is intended for use by
post-ROM boot stage components, via the ROM Vector Table."

Once invoked the part will drop down to its BootROM USB recovery mode.
Should it be the case that the part is in secure boot mode - only an
appropriately signed binary will be accepted by the ROM and subsequently
executed.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Sven Ebenfeld <sven.ebenfeld@gmail.com>
Cc: George McCollister <george.mccollister@gmail.com>
Cc: Breno Matheus Lima <brenomatheus@gmail.com>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>