spiffs: Make spiffs runnable on host

Makes spiffs component runnable on host. Depends on the host library build
of flash emulator. Includes a basic sanity test of
mounting a volume, opening a file, writing to the file, reading the file,
closing the file and unmounting volume.

components/spiffs/test_spiffs_host/main.cpp

@@ -0,0 +1,3 @@
+#define CATCH_CONFIG_MAIN
+#include "catch.hpp"
+