Tockloader

In this release:

- Preliminary support for listening to `printf()` from a board through Segger's JLinkRTTClient tool. This requires that the JLink tools be installed. `tockloader listen --jlink` to use.
- Add nrf51dk, launchxl-cc26x2r1, and ek-tm4c1294xl as known boards.
- Allow apps to be installed from URLs.

This release brings Tockloader 1.0! The basic UI is now stable (install, uninstall, etc.). The newer functions are not guaranteed to be stable, but there are likely few if any people using them regularly.

Changes

- `--jtag` is deprecated and replaced with `--jlink`.
- Tockloader can parse the crt0 header used by C apps.
- Added `--no-bootloader-entry` to tell tockloader to assume the board is already in bootloader mode.
- Added support for chip specific flash page sizes.
- `listen` can print timestamps and line counts.
- `install` can fetch apps from a server.
- Added support for OpenOCD.

- The command `dump-flash-page` can now accept a page number in hexadecimal.
- New command called `read` for reading arbitrary ranges of flash memory.
- An interrupted `listen` session will now resume listening on the same serial device as it started on. This is relevant when there are multiple bootloader supporting boards attached.

And these bug fixes:

- Escape characters in the serial bootloader are now handled properly. Before they could be escaped multiple times.
- App binaries are now checked to ensure they aren't longer than their header says they should be.
- Flags specified before/after the command are handled properly.

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.

- Slight change to TBF header (now padded to 4 bytes).
- Also don't change baud rate unless told to. This needs more testing.

Since we use an invalid header to mark the end of apps, tockloader was always printing warnings about unknown TBF versions. This removes that warning printout.