configure: List available targets in --help output Include the list of available targets in the --help output for the --target-list= option. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Acked-by: Stefan Weil <weil@mail.berlios.de> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>