scripts: Add sort-makefile-lines.py to sort Makefile variables.

The scripts/sort-makefile-lines.py script sorts Makefile variables
according to project expected order.

The script can be used like this:

$ scripts/sort-makefile-lines.py < elf/Makefile > elf/Makefile.tmp
$ mv elf/Makefile.tmp elf/Makefile

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>