samples: i2s_codec: Reorganise Kconfig fragments

Reorganise Kconfig fragments (prj.conf, boards/*.conf). Rename
mimxrt1170_evk@B/mimxrt1176/cm7 fragments to
mimxrt1170_evk/mimxrt1176/cm7. Change Kconfig defaults for SAMPLE_RATE
and I2S_INIT_BUFFER.

This change is meant to reduce the length of target-specific
configuration, as some of the settings are duplicated across targets.
Also, with the inclusion of the sine wave vector, the default sample
rate of this sample changed to 16 kHz.

Signed-off-by: Vit Stanicek <vit.stanicek@nxp.com>