Add feature for building with std support

[CliffHan]

Added esp32-nimble into an esp32 std project and failed to build. That's because critical-section used a built-in critical section implementation when std enabled. So I think it's better to add a feature in this project to make it work with std.

[taks]

Thanks! In other libraries, the default feature is std. Please change this pull request in the same way.

[CliffHan]

Patch updated.

[taks]

Thanks for the change.