boards: atmel: set BUILD_OUTPUT_HEX for OpenOCD boards

The 3124c029 introduced the requirement of CONFIG_BUILD_OUTPUT_HEX be on
the boards' default configuration in order for the runner to be passed
the path to the hex file being loaded. Add missing configuration for
Atmel boards that uses OpenOCD for flash and debug.

Fixes #28337.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>