boards: common: programatically set MAX32_TARGET_CFG

Configuration files such as max32655.cfg is used for openocd purpose
and is stored in variable ${MAX32_TARGET_CFG}. It was set manually
based on whether CONFIG_SOC_MAX32655_M4 is set, now, it gets set
automatically utilizing ${CONFIG_SOC} variable.

Signed-off-by: Hao Zhang <Hao.Zhang@analog.com>