bootconfig: Add append value operator support

Add append value operator "+=" support to bootconfig syntax.
With this operator, user can add new value to the key as
an entry of array instead of overwriting.
For example,

  foo = bar
  ...
  foo += baz

Then the key "foo" has "bar" and "baz" values as an array.

Link: http://lkml.kernel.org/r/158227283195.12842.8310503105963275584.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

1 files changed, 9 insertions, 1 deletions

diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst
index 57119fb69d36..cf2edcd09183 100644
--- a/Documentation/admin-guide/bootconfig.rst
+++ b/Documentation/admin-guide/bootconfig.rst
@@ -71,7 +71,15 @@ For example,::
  foo = bar, baz
  foo = qux  # !ERROR! we can not re-define same key
 
-Also, a sub-key and a value can not co-exist under a parent key.
+If you want to append the value to existing key as an array member,
+you can use ``+=`` operator. For example::
+
+ foo = bar, baz
+ foo += qux
+
+In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
+
+However, a sub-key and a value can not co-exist under a parent key.
 For example, following config is NOT allowed.::
 
  foo = value1