tools/bootconfig: Support mixed value and subkey test cases

Update test case to support mixed value and subkey on a key.

Link: https://lkml.kernel.org/r/162262195568.264090.12431204030021242896.stgit@devnote2Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

tools/bootconfig/samples/bad-override.bconf

-key.subkey = value
-# We can not override pre-defined subkeys with value
-key := value

tools/bootconfig/samples/bad-override2.bconf

-key = value
-# We can not override pre-defined value with subkey
-key.subkey := value

tools/bootconfig/samples/good-mixed-append.bconf

+key = foo
+keyx.subkey = value
+key += bar
+

tools/bootconfig/samples/good-mixed-kv3.bconf

+# mixed key and subkeys with braces
+key = value
+key {
+	subkey1
+	subkey2 = foo
+}

tools/bootconfig/samples/good-mixed-override.bconf

+key.foo = bar
+key = value
+# mixed key value can be overridden
+key := value2