xbps-src: added suport for XBPS_CACHEDIR in configuration file.

XBPS_CACHEDIR will be bind mounted as read/write in target masterdir
into /cachedir. This directory must have read/write perms for the user
running xbps-src.

xbps-src/etc/xbps-src.conf.in

@@ -4,7 +4,8 @@
 
 #
 # Directory where the xbps-src GIT repository is stored. This is
-# required to be mounted in the chroot.
+# required to be mounted in the chroot. You shouldn't need to modify this
+# one unless your vanilla/xbps-templates repo has been moved to another path.
 #
 XBPS_DISTRIBUTIONDIR=@@XBPS_DISTRIBDIR@@
 
@@ -13,6 +14,12 @@ XBPS_DISTRIBUTIONDIR=@@XBPS_DISTRIBDIR@@
 #
 XBPS_MASTERDIR=$XBPS_DISTRIBUTIONDIR/masterdir
 
+#
+# Cache directory for downloaded binary packages; it's bind mounted
+# to $XBPS_MASTERDIR/cachedir for chroot operations.
+#
+#XBPS_CACHEDIR=/path/to/cachedir/with/user/permissions/to/write
+
 #
 # Compilation flags for cc and c++.
 #