Hello Aykut Birsen,
I am also having same problem of booting from external serial flash.
I am using SST25VF040B serial flash.
I have connected SPI1_DATIN Pin to the ground with 100K resistence. but still its not taking code from serial flash.
But with osilloscope i can see the out put waveforms comming out from ext serial flash's pin 2 which is SPI_DATA_OUT. after the reset.
could you please help me with circuit(with SPI1_DATIN Pin)connections and that code which u loaded into ext serial flash. for my reference.
Thanks in adavance.