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>

plat/imx/imx7/warp7/aarch32/warp7_helpers.S

@@ -8,11 +8,13 @@
 #include <asm_macros.S>
 #include <assert_macros.S>
 #include <platform_def.h>
+#include <imx_hab.h>
 
 	.globl	platform_mem_init
 	.globl	plat_get_my_entrypoint
 	.globl	plat_crash_console_init
 	.globl	plat_crash_console_putc
+	.globl	plat_panic_handler
 
 	/* ---------------------------------------------
 	 * int plat_mem_init(void)
@@ -42,3 +44,9 @@ func plat_crash_console_putc
 	mov_imm r1, PLAT_WARP7_BOOT_UART_BASE
 	b	imx_crash_uart_putc
 endfunc plat_crash_console_putc
+
+func plat_panic_handler
+	mov	r3, #HAB_ROM_VECTOR_TABLE_FAILSAFE
+	ldr	r3, [r3, #0]
+	blx	r3
+endfunc plat_panic_handler