__FreeBSD_version: Bump for mkimg -f vmdks

[cperciva]

Summary

Also, adjust bootstrap tools code in Makefile.inc1 to reflect this; in
addition to rebuilding mkimg if it is out of date, we now need to build
libz, which was not previously needed by mkimg.

Sponsored by: Amazon

[cperciva]

@kevans You've touched the bootstrap tools code in Makefile.inc1 recently; am I doing this right?

[kevans]

Yes, this is perfectly sensible.