vfs: define all implementations as esp_vfs_, create aliases
vfs: define all implementations as esp_vfs_, create aliases

This change allows (in the future) to test VFS on host, without having
it conflict with the host C library. On host, all aliases would be
disabled.

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf