Apply r346792 (cperciva) from stable/12 to head. The original commit

message:

 On non-x86 systems, use "quarterly" packages.

 x86 architectures have "latest" package builds on stable/*, so keep using
 those (they'll get switched over to "quarterly" during releases).

The original commit was a direct commit to stable/12, as at the time it
was presumed it would not be necessary for head.  However, when it is time
to create a releng branch or switch from PRERELEASE/STABLE to BETA/RC, the
pkg(7) Makefile needs further adjusting.  This commit includes those
further adjustments, evaluating the BRANCH variable from release/Makefile
to determine the pkg(7) repository to use.

MFC after:	immediate (if possible)
Sponsored by:	Rubicon Communications, LLC (Netgate)

usr.sbin/pkg/Makefile

@@ -1,6 +1,16 @@
 # $FreeBSD$
 
+.if ${MACHINE} != "amd64" && ${MACHINE} != "i386"
+PKGCONFBRANCH?=	quarterly
+.else
+_BRANCH!=	${MAKE} -C ${SRCTOP}/release -V BRANCH
+BRANCH?=	${_BRANCH}
+. if ${BRANCH:MBETA*} || ${BRANCH:MRC*} || ${BRANCH:MRELEASE*}
+PKGCONFBRANCH?=	quarterly
+. else
 PKGCONFBRANCH?=	latest
+. endif
+.endif
 CONFS=		FreeBSD.conf.${PKGCONFBRANCH}
 CONFSNAME=	FreeBSD.conf
 CONFSDIR=	/etc/pkg