Modified line 145:  previously the fill character has been hard coded to 0x00. However in Mbed OS core SPI_FILL_CHAR is defined in the core to be 0xFF by default (one can change that through mbed_app.json for example). This mod allows us to use the same fill character that is defined for Mbed OS.

Also modified spi_master_block_write(): previously it called am_hal_iom_blocking_transfer on line 182, but that prevented succesful SD card writing operations. Now i changed that part to am_hal_iom_spi_blocking_fullduplex and SD functionality seems to be working.