Add WASI support for esp-idf platform:
1. add Kconfig and cmake scripts
2. add API "openat" when using littlefs
3. add clock/rwlock/file/socket OS adapter
Various fixes and beautifications coordinated with @1c3t3a,
fixes 2 of the 3 all remaining issues from #892:
- enable to os_mmap executable memory
- fix os_malloc/os_realloc/os_free issues
- implement os_thread_get_stack_boundary
- add build scripts to include with esp-idf to use wamr as
an ESP-IDF component
- update sample and document
