xbps-src: infrastructure changes required for "nonfree" keyword in tmpls.

- The chroot target now execs masterdir/bin/xbps-shell to set up correct envvars.
- 2 pkgs are now in nonfree repo: adobe-flash-plugin and jre.
- Renamed some vars in xbps-src to be in uppercase, notably XBPS_MACHINE.
- make-repoidx target now creates the index for all "known" directories specified
  via XBPS_PACKAGESDIR: XBPS_MACHINE, noarch, and nonfree/XBPS_MACHINE.

srcpkgs/zlib/template

@@ -5,7 +5,7 @@ revision=4
 distfiles="http://www.zlib.net/$pkgname-$version.tar.bz2"
 build_style=configure
 configure_args="--prefix=/usr --shared"
-if [ "${xbps_machine}" = "x86_64" ]; then
+if [ "${XBPS_MACHINE}" = "x86_64" ]; then
 	configure_args="${configure_args} --64"
 fi
 make_build_args="CFLAGS=-fPIC"