target/avr: Introduce enumeration AVRFeature

target/avr: Introduce enumeration AVRFeature

This patch introduces enumeration "AVRFeature" that will be
used for defining various AVR core types.

[AM: Split a larger AVR introduction patch into logical units]
Suggested-by: Aleksandar Markovic <[email protected]>
Co-developed-by: Michael Rolnik <[email protected]>
Co-developed-by: Sarah Harris <[email protected]>
Signed-off-by: Michael Rolnik <[email protected]>
Signed-off-by: Sarah Harris <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Signed-off-by: Aleksandar Markovic <[email protected]>
Acked-by: Igor Mammedov <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <20200705140315[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>