storage: Add STORAGE_HOSTFS

CONFIG_STORAGE & STORAGE_HOSTFS allows to use parts of the storage_* API to be
compiled for application targets without compiling storage.c or performing
actually raw storage access. This is primarily to enable application targets to
implement HAVE_MULTIVOMULE/HAVE_HOTSWAP (in a later commit).

SIMULATOR uses the same mechanism without explicitely defining STORAGE_HOSTFS
(how to add a bit to an existing preprocessor token?).

Change-Id: Ib3f8ee0d5231e2ed21ff00842d51e32bc4fc7292

firmware/target/hosted/sdl/system-sdl.c

@@ -24,6 +24,9 @@
 #include <stdlib.h>
 #include <string.h>
 #include <inttypes.h>
+#ifdef __unix__
+#include <unistd.h>
+#endif
 #include "system.h"
 #include "kernel.h"
 #include "thread-sdl.h"
@@ -286,6 +289,21 @@ void system_exception_wait(void)
     system_reboot();
 }
 
+void hostfs_init(void)
+{
+    /* stub */
+}
+
+#ifdef HAVE_STORAGE_FLUSH
+int hostfs_flush(void)
+{
+#ifdef __unix__
+    sync();
+#endif
+    return 0;
+}
+#endif /* HAVE_STORAGE_FLUSH */
+
 void sys_handle_argv(int argc, char *argv[])
 {
     if (argc >= 1)