mb/purism/librem_mini: Implement `die_notify`

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>