Need to exclude some ./flash.sh commands for pandaboard

Pandaboards are special. You have to manually put the pandaboard in fastboot mode because any 'reboot' command causes it to just shutdown and not reboot, so I had to change some things in ./flash.sh:

- remove reboot bootloader code,
- erase userdata takes eons to complete and isn't necessary, so i removed it
- update_time doesn't work because you can't reboot the panda this way. It just shuts down, so i removed it for pandas

I tested this flash.sh file against a pandaboard and for an unagi build, and it works as expected. The Unagi build runs all the previous commands, and the pandabuild excludes the ones in the if statements.