mb/purism/librem_mini: Implement `die_notify`

Make the SATA LED blink when coreboot dies. GPIO functions aren't
compiled in for postcar, so add a check to prevent linker failures.

TEST: Try to boot Librem Mini WHL without RAM, observe blinking (and
      also blinding LED). Re-install RAM (and re-seat RAM a few times),
      boot to OS, and observe SATA LED operating normally, as expected.

Change-Id: I0ffac0ab02e52e9fbba7990f401d87e50a1b5154
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50013
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Benjamin Doron <benjamin.doron00@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/purism/librem_cnl/Makefile.inc

@@ -7,3 +7,5 @@ ramstage-y += variants/$(VARIANT_DIR)/hda_verb.c
 ramstage-y += ramstage.c
 
 CPPFLAGS_common += -I$(src)/mainboard/$(MAINBOARDDIR)/variants/$(VARIANT_DIR)/include
+
+subdirs-y += variants/$(VARIANT_DIR)/

src/mainboard/purism/librem_cnl/variants/librem_mini/Makefile.inc

@@ -0,0 +1,4 @@
+## SPDX-License-Identifier: GPL-2.0-only
+
+all-y += die.c
+smm-y += die.c

src/mainboard/purism/librem_cnl/variants/librem_mini/die.c

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <console/console.h>
+#include <delay.h>
+#include <gpio.h>
+#include <soc/gpio.h>
+
+void die_notify(void)
+{
+	if (ENV_POSTCAR)
+		return;
+
+	/* Make SATA LED blink */
+	gpio_output(GPP_E8, 1);
+
+	while (1) {
+		gpio_set(GPP_E8, 0);
+		mdelay(100);
+		gpio_set(GPP_E8, 1);
+		mdelay(100);
+	}
+}