fix genfilelist

Fixed the sort for `make genfilelist` to ignore case
and to use the ${SORT} makefile variable.

2020/.filelist

@@ -1,6 +1,4 @@
 2020/2020.tar.bz2
-2020/Makefile
-2020/README.md
 2020/.filelist
 2020/.gitignore
 2020/guidelines.txt
@@ -8,5 +6,7 @@
 2020/iocccsize.c
 2020/iocccsize.mk
 2020/iocccsize-test.sh
+2020/Makefile
+2020/README.md
 2020/rules.txt
 2020/.year