Kconfig: enable BUILD_OUTPUT_HEX for sam_e70_xplained

now we use hex file instead of elf file for flash command as
PR #17822 suggested, so enable BUILD_OUTPUT_HEX by default for
board sam_e70_xplained.

Fixes: #18181.

Signed-off-by: Wentong Wu <wentong.wu@intel.com>