add more built-in commands

[vulcu]

possible ideas:

• set port pin as output
  • set pin state high/low
  • set pin to pulse low/high/low or high/low/high for a given pulse width
  • set a PWM value
• set port pin as input
  • read input state
  • enable/disable input pullup
  • read ADC value of analog input pins
• show all port values and current state (input/input_pullup/output)
• SPI
  • assign a CS pin and name to an SPI device
  • perform a SPI read/write (similar to existing I2C) using a named device
  • delete a named SPI device
• EEPROM
  • save current pin configuration to EEPROM
  • save SPI config to EEPROM
  • load pin and SPI configuration from EEPROM
  • erase EEPROM
    • erase all pin configurations
    • erase all SPI devices
  • set EEPROM value
  • read EEPROM value
• reboot/reset system (depends on how portable this is)