framework: cleanup conditional-indentations in Mk/

Run Tools/scripts/indent_make_if.pl on all of Mk.

These white space changes contribute greatly to the readability of those files.
As we have a version control system, finding out the reasons for the changes
prior to these white space changes is still easily possible

Differential Revision: https://reviews.freebsd.org/D35024
Reviewed by: portmgr (rene, bapt)