release: install pkg to install media

This is the most straightforward way to make installation of base system
packages from install media possible. Bootstrapping pkg at install time
does not work since the root filesystem of install media is immutable.

Reviewed by:	bz, cperciva, emaste
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49968

release/Makefile

@@ -193,6 +193,7 @@ disc1: packagesystem
 .endif
 .if ${.MAKE.OS} == "FreeBSD" && (!defined(NOPKG) || empty(NOPKG))
 # Install packages onto release media.
+	${PKG_INSTALL} pkg || true
 	${PKG_INSTALL} wifi-firmware-kmod-release || true
 	${PKG_CLEAN} || true
 .endif
@@ -231,6 +232,11 @@ bootonly: packagesystem
 	cp MANIFEST ${.TARGET}/usr/freebsd-dist
 	echo "./usr/freebsd-dist type=dir uname=root gname=wheel mode=0755" >> ${.TARGET}/METALOG
 	echo "./usr/freebsd-dist/MANIFEST type=file uname=root gname=wheel mode=0644" >> ${.TARGET}/METALOG
+.if ${.MAKE.OS} == "FreeBSD" && (!defined(NOPKG) || empty(NOPKG))
+# Install packages onto release media.
+	${PKG_INSTALL} pkg || true
+	${PKG_CLEAN} || true
+.endif
 # Set up installation environment
 	ln -fs /tmp/bsdinstall_etc/resolv.conf ${.TARGET}/etc/resolv.conf
 	echo sendmail_enable=\"NONE\" > ${.TARGET}/etc/rc.conf
@@ -266,6 +272,7 @@ dvd: packagesystem
 .endif
 .if ${.MAKE.OS} == "FreeBSD" && (!defined(NOPKG) || empty(NOPKG))
 # Install packages onto release media.
+	${PKG_INSTALL} pkg || true
 	${PKG_INSTALL} wifi-firmware-kmod-release || true
 	${PKG_CLEAN} || true
 .endif