u-boot中CONFIG_SUPPORT_SPL、CONFIG_SPL和CONFIG_SPL_BUILD的作用

u-boot关于SPL有三个配置项，CONFIG_SUPPORT_SPL、CONFIG_SPL和CONFIG_SPL_BUILD，其中CONFIG_SUPPORT_SPL和CONFIG_SPL在Kconfig中定义，如下：

config SUPPORT_SPL
        bool

config SUPPORT_TPL
        bool

config SPL
        bool
        depends on SUPPORT_SPL
        prompt "Enable SPL"
        help
          If you want to build SPL as well as the normal image, say Y.

CONFIG_SUPPORT_SPL表示是否支持SPL功能，一般是在具体arch的Kconfig的版型菜单中，根据需要select