sandbox: implement reset

Up to now the sandbox would shutdown upon a cold reset request. Instead it
should be reset.

In our coding we use static variables like LIST_HEAD(efi_obj_list). A reset
can occur at any time, e.g. via an UEFI binary calling the reset service.
The only safe way to return to an initial state is to relaunch the U-Boot
binary.

The reset implementation uses execv() to relaunch U-Boot.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/sysreset/sysreset_sandbox.c

@@ -56,6 +56,9 @@ static int sandbox_sysreset_request(struct udevice *dev, enum sysreset_t type)
 	switch (type) {
 	case SYSRESET_COLD:
 		state->last_sysreset = type;
+		if (!state->sysreset_allowed[type])
+			return -EACCES;
+		sandbox_reset();
 		break;
 	case SYSRESET_POWER_OFF:
 		state->last_sysreset = type;