Support for `appaddr` attribute

This release:

• Adds support for determining where the start of applications is using a bootloader attribute called appaddr. This is considered temporary until kernel attributes are supported and this setting will be moved there.
• Includes some known Tock supporting boards so that if you pass --board Tockloader will use the known boards to fill in --arch and --jtag-device.
• Includes more debugging help when the necessary board properties aren't passed to Tockloader.