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.htmlSuggested-by: NRob Clark <robdclark@gmail.com>
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>