MFC r287696:
The <arch>/mkisoimages.sh script in release knows how to add
extra bits from an "xtra-bits-dir". This feature is unusable
from release/Makefile. Add an XTRADIR setting to use it.
MFC r287697: Whitespace alignment

Differential Revision: https://reviews.freebsd.org/D3633
Reviewed by: kmacy
Relnotes: yes