Don't install src while creating box
This speeds up box creation a lot.  The box creation is using pkg(1)
to install software so doesn't need source files.  It doesn't make much
sense installing all the source files and then deleting them during
cleanup stage.
MODIFY
freebsd/http/freebsd-11/installerconfig
freebsd/http/freebsd-11/installerconfig
-DISTRIBUTIONS="base.txz lib32.txz kernel.txz src.txz"
+DISTRIBUTIONS="base.txz lib32.txz kernel.txz"
# for variations in the root disk device name between VMware and Virtualbox
if [ -e /dev/ada0 ]; then

MODIFY
freebsd/scripts/cleanup.sh
freebsd/scripts/cleanup.sh
@@ -7,6 +7,5 @@ rm -f /var/db/freebsd-update/*-rollback;
rm -rf /var/db/freebsd-update/install.*;
rm -rf /boot/kernel.old;
rm -f /boot/kernel*/*.symbols;
-rm -rf /usr/src/*;
rm -f /*.core;
rm -rf /var/cache/pkg;

Don't install src while creating box

This speeds up box creation a lot.  The box creation is using pkg(1)
to install software so doesn't need source files.  It doesn't make much
sense installing all the source files and then deleting them during
cleanup stage.