xbps-src: new layout in masterdir to use a common directory from host.

XBPS_CACHEDIR has been replaced by XBPS_HOSTDIR in configuration file,
and this expects the following structure:

/host
  |_ /build (previously /pkg-builddir)
  |_ /binpkgs (previously /pkg-binpkgs)
  |_ /repocache (previously /cachedir)
  |_ /sources (previously /pkg-srcdistdir)

Thanks to str1ngs for the great idea!

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

@@ -15,10 +15,23 @@ 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.
+# Host directory to be bind mounted into the chroot (masterdir) containing
+# directories for downloaded cached packages from xbps-bin(8), packages
+# created by xbps-src, source distribution tarballs downloaded required
+# to build packages from source, etc.
 #
-#XBPS_CACHEDIR=/path/to/cachedir/with/user/permissions/to/write
+# Basically your directory must contain this structure:
+#
+# /yourdir
+# 	|_ /build
+# 	|_ /binpkgs
+# 	|_ /repocache
+# 	|_ /sources
+#
+# Those names are constants so you cannot change them, even they are
+# case sensitive.
+#
+#XBPS_HOSTDIR=/path/to/your/host/directory
 
 #
 # Compilation flags for C and C++.
@@ -60,7 +73,7 @@ XBPS_COMPRESS_LEVEL=9
 
 #
 # Install required build time dependencies from binary packages,
-# Local repository associated with the masterdir at <masterdir>/pkg-binpkgs,
+# Local repository associated with the masterdir at <masterdir>/host/binpkgs,
 # and remote repositories specified in XBPS_REPO_LIST will be used
 # to resolve build dependencies.
 #