* b/piboot.go: check EEPROM version for RPi4
Check EEPROM version for RPi4 to make sure that tryboot is
supported. If the EEPROM need updating, return an error when
installing the kernel.
* bootloader: add unit tests for detection of RPi4 EEPROM
* bootloader/piboot: use osutil instead of using directly os.Stat
* bootloader: address review comments