kconfig: fix new choices being skipped upon config update

Running `oldconfig' after any of the following configuration change:

either trivial addition, such as:

config A
	bool "A"

choice
	prompt "Choice ?"
	depends on A

	config CHOICE_B
		bool "Choice B"

	config CHOICE_C
		bool "Choice C"
endchoice

or more tricky change:

OLD KCONFIG                      |  NEW KCONFIG
                                 |
                                 |  config A
                                 |          bool "A"
                                 |
choice                           |  choice
        prompt "Choice ?"        |          prompt "Choice ?"
                                 |
        config CHOICE_C          |          config CHOICE_C
                bool "Choice C"  |                  bool "Choice C"
                                 |
        config CHOICE_D          |          config CHOICE_D
                bool "Choice D"  |                  bool "Choice D"
endchoice                        |
        ...