efi_loader: manage events in a linked list

Lift the limit on the number of events by using a linked list.

This also allows to have events with type == 0.

This patch is based on Rob's patch
efi_loader: fix events
https://lists.denx.de/pipermail/u-boot/2017-October/309348.html



Suggested-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>