hosted: Use O_CLOEXEC for all open() and "e" for fopen() calls

This way we'll automatically close the files upon exec()

Change-Id: Ic0daca8fb56432830de4a2f4a86a77337121ecc7

firmware/target/hosted/filesystem-app.c

@@ -234,6 +234,7 @@ int app_open(const char *path, int oflag, ...)
     if (!fpath)
         FILE_ERROR_RETURN(ENAMETOOLONG, -1);
 
+    oflag |= O_CLOEXEC;
     return os_open(fpath, oflag __OPEN_MODE_ARG);
 }