cmake:fix invalid configs when export defconfig and .config

[xuxin930]

Summary

when configuring nuttx compile-time config, defconfig and .config need to be read,
and the func nuttx_export_kconfig will be called twice.

this will cause dirty data in cmake config, resulting in unexpected compilation problems.

# for example my defconfig enable CONFIG_FEAT_FOO
# then call cmake --build build menuconfig
#         disable CONFIG_FEAT_FOO
# variables in the cmake compilation phase CONFIG_FEAT_FOO still y
# this creates compilation issues

this patch records the key values modified each time the config is refreshed, and then deletes these keys when calling nuttx_export_kconfig again, thereby updating the global config.

similar to the CONFIG_BASE_DEFCONFIG flag -dirty in the Makefile.

Impact

Testing

• CI Build
• cmake --build build -t menuconfig # disable some feature

[anchao]

LGTM