docs: bootconfig: Update for mixing value and subkeys

In this case, the key ``foo`` has ``bar``, ``baz`` and ``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.

Moreover, sub-keys and a value can coexist under a parent key.

For example, following config is NOT allowed.::

For example, following config is allowed.::

 foo = value1

 foo = value1

 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist

 foo.bar = value2

 foo.bar := value2 # !ERROR! even with the override operator, this is NOT allowed.

 foo := value3 # This will update foo's value.

Note, since there is no syntax to put a raw value directly under a

structured key, you have to define it outside of the brace. For example::

 foo {

     bar = value1

     bar {

         baz = value2

         qux = value3

     }

 }

Also, the order of the value node under a key is fixed. If there

are a value and subkeys, the value is always the first child node

of the key. Thus if user specifies subkeys first, e.g.::

 foo.bar = value1

 foo = value2

In the program (and /proc/bootconfig), it will be shown as below::

 foo = value2

 foo.bar = value1

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