efi_loader: non-volatile variable not deleted from file

When deleting a non-volatile variable it was deleted from memory but the
deletion was not persisted to the file system.

SetVariable() may be called with attributes == 0 to delete a variable. To
determine if the deletion shall be persisted we have to consider the
non-volatile flag in the attributes of the deleted variable and not the
value passed in the call parameter.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>