fix infinite loop bug when HIGH-Z MISO stays high

When FLASH chip is not present and MISO is in high impedance, other SPI
devices might pull MISO high and the while(busy) loop might never return
because of a false positive busy (actually MISO would return 0XFF for
status register including the BUSY bit)
So because bootloader space is very limited a simple JEDEC_ID read is
done to check for flash chip presence