efi_selftest: make tests easier to read

efi_selftest: make tests easier to read

Rename counter to more illustrative names.
Update notification function description.
Simplify notification function.
Add comment for arbitrary non-zero value.
Document @return.
Use constants for return values of setup, execute, teardown.

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