dev-manual: Updated EXTRA_IMAGE_FEATURES operater for read-only-rootfs example

The operator used was "=" which was inconsistent in light of
previous uses of the variable in the ptest section.  I changed the
operator to "+=" to be consistent.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
(From yocto-docs rev: d6b7c7e15290e64f4baac25544224788e92ad309)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/documentation/dev-manual/dev-manual-common-tasks.xml b/documentation/dev-manual/dev-manual-common-tasks.xml
index 7138f5dca6..8d251a0125 100644
--- a/documentation/dev-manual/dev-manual-common-tasks.xml
+++ b/documentation/dev-manual/dev-manual-common-tasks.xml
@@ -4435,7 +4435,7 @@
                 </literallayout>
                 or
                 <literallayout class='monospaced'>
-     EXTRA_IMAGE_FEATURES = "read-only-rootfs"
+     EXTRA_IMAGE_FEATURES += "read-only-rootfs"
                 </literallayout>
             </para>