Do not initialize RootPosition at startup

Initializing high-level object at startup is very dangerous,
because low-level snippets are not yet initialized.

For example Position's constructor calls find_checkers() which
calls attackers_to() which depends on various global bitboard arrays
which are not yet initialized. I think we are lucky not to crash.

RootPosition.from_fen(StartPosition); is called immediately after
all initializations are made at uci_main_loop() which is the
correct behaviour

Signed-off-by: Marco Costalba <mcostalba@gmail.com>