pkgbootstrap: Gate building from ports

In release builds we want to build pkg from the ports tree for reasons
mentioned in a previous commit; but non-release pkgbase builds may be
just fine with installing pkg from pkg.freebsd.org.

Gate it behind BOOTSTRAP_PKG_FROM_PORTS rather than building form ports
any time we have a ports tree available.

While I'm here, make the pkgbase-repo target in release/Makefile
create the temporary directories we need for building.

Reviewed by:	ivy
MFC after:	6 hours
Sponsored by:	https://www.patreon.com/cperciva
Fixes:	2c06b82443 ("pkgbootstrap: Build pkg if PORTSDIR exists")

Makefile.inc1

@@ -1992,7 +1992,7 @@ PKG_ABI!=	${PKG_CMD} -o ABI_FILE=${PKG_ABI_FILE} config ABI
 
 _pkgbootstrap: .PHONY
 .if make(*package*) && !exists(${LOCALBASE}/sbin/pkg)
-.if exists(${PORTSDIR}/ports-mgmt/pkg/Makefile)
+.if defined(BOOTSTRAP_PKG_FROM_PORTS)
 	env - UNAME_r=${UNAME_r} PATH=$$PATH \
 	    make -C ${PORTSDIR}/ports-mgmt/pkg \
 	    I_DONT_CARE_IF_MY_BUILDS_TARGET_THE_WRONG_RELEASE=1 \

release/Makefile

@@ -195,8 +195,9 @@ PKG_ABI=	$$(${PKG_CMD} -o ABI_FILE=${.TARGET}/usr/bin/uname config ABI)
 
 pkgbase-repo:
 	mkdir -p pkgbase-repo
+	mkdir -p /tmp/ports /tmp/distfiles
 	( ${IMAKE} -C ${WORLDDIR} packages REPODIR=${.OBJDIR}/pkgbase-repo \
-	    INCLUDE_PKG_IN_PKGBASE_REPO=YES )
+	    INCLUDE_PKG_IN_PKGBASE_REPO=YES BOOTSTRAP_PKG_FROM_PORTS=YES )
 
 disc1: ${PKGBASE_REPO}
 # Install system