vmimage.subr: Add ability to install src in VM image

In some cases having a src tree in a VM image is convenient
for development or debugging. Add a WITH_SRC variable,
which, when set, will cause the vm-release target to include
FreeBSD-set-src in the list of packages installed in an image.

Signed-off-by: Krzysztof Galazka <krzysztof.galazka@intel.com>

Sponsored by:   Intel Corporation

Reviewed by:	cperciva
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D57143

release/tools/vmimage.subr

@@ -81,6 +81,7 @@ vm_base_packages_list() {
 		echo FreeBSD-set-lib32
 		[ -z "${WITHOUT_DEBUG_FILES}" ] && echo FreeBSD-set-lib32-dbg
 	esac
+	[ -n "${WITH_SRC}" ] && echo FreeBSD-set-src
 	echo FreeBSD-set-tests
 	# Also install pkg, since systems with a packaged base system should
 	# have the tools to upgrade themselves.