Allow emulation of bootloader

[jbush001]

In emulator, add option to create a TTY that is connected to the serial port, so the serial_boot program can run against it and test uploading a program.

https://gist.github.com/jbush001/c69699145c17597082c06566b8fcde15

[jbush001]

This would also require emulating the ROM in high memory. The emulator would need to read a bootrom file at startup, and make memory accesses to that region return values from it.