Added the ravedude chip command for bare MCU development

[innermatrix]

This PR adds a ravedude chip command, aimed primarily at anyone doing bare chip development. The behavior of the ravedude chip command is as follows:

1. ravedude chip can be used as a cargo runner
2. The target MCU is autodetected from the ELF metadata in the binary that is being flashed. See ravedude/src/target_detect.rs.
3. The programmer is specified either with command line options or with environment variables. There is no provision for specifying the programmer using a configuration file. The rationale for this is

• The programmer and its associated settings (port and baud rate) are not a property of the crate, but are configuration parameters specific to a particular developer and their development system.
• Any crate that wishes to hardcode programmer settings can do so in .cargo/config.toml and doesn't need a separate config file.
• Any crate that wishes to leave programmer settings up to the individual developer can leave them out of .cargo/config.toml and require the developer to configure them with environment variables
• Any developer that wishes to use a file on disk to configure their environment can use direnv or a similar tool.

4. ravedude chip supports opening a serial console after flashing. Serial console settings (port and baud rate) are specified with either command line options or with environment variable. Typically, the baud rate would be specified in .cargo/config.toml (because it's a property of the crate) and the port would be specified in an environment variable (because it's a developer-specific configuration parameter).

In summary, the intended use of ravedude chip is:

• Set runner in .cargo/config.toml to ravedude chip (for no serial console after flashing) or ravedude chip --console-mode=serial --console-baudrate=NNN (for serial console after flashing)
• Set environment variables for AVR_PROGRAMMER_NAME, AVR_PROGRAMMER_PORT (if not autodetected byavrdude), AVR_PROGRAMMER_BAUDRATE (if not autodetected byavrdude), and AVR_CONSOLE_PORT (if different from AVR_PROGRAMMER_PORT) using any of the following methods: directly set them in the shell before cargo run, set them in the [env] section of .cargo/config.toml, or use direnv or a similar tool.

For consistency, the existing ravedude command has been renamed ravedude board; if neither ravedude chip nor ravedude board has been specified, ravedude board is implied. That is, the following work:

• runner = ravedude — works like before
• runner = ravedude board — works same as ravedude
• runner = ravedude chip — new addition, works as described above.

I am keeping ravedude chip out of the main package docs for now because its only imminent use will be internal (for developing inside MCU crates), but I do plan to add more documentation for it in the future (probably when I add examples in MCU crates, which I am working on in a separate branch).