efi_loader: use type bool for event states

Queued and signaled describe boolean states of events.
So let's use type bool and rename the structure members to is_queued
and is_signaled.

Update the comments for is_queued and is_signaled.

Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>