man: Document mask workaround for sysext images

A read-only /usr may ship a sysext image by default and the user wants
to opt out. Currently it's not clear how to do this.
Document that a /dev/null symlink in /etc/extensions/ works to "mask" a
sysext image in a folder with lower precedence.

diff --git a/man/systemd-sysext.xml b/man/systemd-sysext.xml
index d7f5d2f37f89e24098dac5590cb1c5be000ff82c..42e8e12df97ff660c39d4f6171291067f5a21c6b 100644 (file)
--- a/man/systemd-sysext.xml
+++ b/man/systemd-sysext.xml
@@ -108,7 +108,9 @@
     accessed.</para>
 
     <para>Note that there is no concept of enabling/disabling installed system extension images: all
-    installed extension images are automatically activated at boot.</para>
+    installed extension images are automatically activated at boot. However, you can place a symlink
+    to <filename>/dev/null</filename> in <filename>/etc/extensions/</filename> to "mask" an image with
+    the same name in a folder with lower precedence.</para>
 
     <para>A simple mechanism for version compatibility is enforced: a system extension image must carry a
     <filename>/usr/lib/extension-release.d/extension-release.<replaceable>$name</replaceable></filename>