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.

Differential Revision: https://reviews.freebsd.org/D3633
Reviewed by: kmacy
MFC after: 3 weeks
X-MFC-to: stable/10
Relnotes: yes