fw/testfw: Use fw_ram for firmware stack

This means firmware's stack shouldn't be accessible to programs
running in app_mode.

It also means we don't need to take special care of secure_ctx which
can now be an ordinary stack variable.

Nonetheless we zero out secure_ctx after final use and inline some
assembler to zero out the entire fw_ram after use, just before
switching to app_mode.

Signed-off-by: Daniel Lublin <daniel@lublin.se>

hw/application_fpga/fw/testfw/main.c

@@ -187,6 +187,13 @@ int main()
 
 	// Turn on application mode.
 	// -------------------------
+
+	// Set up another stack because fw_ram is not available
+	// anymore in app_mode.
+	asm volatile(
+		"li sp, 0x40006ff0"
+		);
+
 	*switch_app = 1;
 
 	sw = *switch_app;