Consider dedicating some flash for configs

[jonlamb-gh]

So things like IP and MAC address can be changed without re-flashing firmware.
Add some config update commands to the CLI for setting them.
Other potentially useful things to have:

• max log level (i.e. trace vs info)
• last panic message
• last reset reason from panic
• env-config constants are used to build up the default config struct values

[jonlamb-gh]

Boot and application configs in sector 3 (0x0800_C000) as of adb1573
Not currently exposing any configs yet, just bootloader flags

[jonlamb-gh]

The bootloader is pretty small, it can fit in sectors 0 and 1, could dedicate sector 2 to bootloader-specific configs, and sector 3 to application-specific configs.

[jonlamb-gh]

Probably use something like TicKV from Tock for a kv store.