Introduce a way to override dependencies for all pkgs thru a file.

The `etc/defaults.virtual` file contains the default replacement
for our use cases; at this moment we only use it to use `eudev`
rather than `systemd` for udev.

If `etc/virtual` exists it overrides the settings of `etc/defaults.virtual`.

README.md

@@ -12,13 +12,15 @@ To start using it first you'll need some external dependencies:
 Make sure your user is added to the `xbuilder` group to be able to use `xbps-uchroot(8)`,
 otherwise `xbps-src` won't work correctly.
 
-The `xbps-src` utility will allow you to generate XBPS binary packages, type
+Type:
 
      $ ./xbps-src -h
 
 to see all available targets/options and start building any available package
 in the `srcpkgs` directory.
 
+### Configuration
+
 The `etc/defaults.conf` file contains the possible settings that can be overrided
 through the `etc/conf` configuration file for the `xbps-src` utility; if that file
 does not exist, will try to read configuration settings from `~/.xbps-src.conf`.
@@ -29,5 +31,15 @@ those defined in `etc/defaults.conf`, append to them instead via `etc/conf` i.e:
     $ echo 'XBPS_CFLAGS+=" your flags here "' >> etc/conf
     $ echo 'XBPS_LDFLAGS+=" your flags here "' >> etc/conf
 
+#### Virtual packages
+
+The `etc/defaults.virtual` file contains the default replacements for virtual packages,
+used as dependencies in the source packages tree.
+
+If you want to customize those replacements, copy `etc/defaults.virtual` to `etc/virtual`
+and edit it accordingly to your needs.
+
+### Contributing
+
 See [Manual](https://github.com/voidlinux/xbps-packages/blob/master/Manual.md)
 for documentation to create and learn about the source packages.