Convert CONFIG_CMD_SCSI to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SCSI

Also update the Makefile to use CONFIG_CMD_SCSI instead of CONFIG_SCSI to
enable the command, fixing an earlier error.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
[trini: Rework to default y if SCSI, drop cl-som-am57x which did not use
CMD_SCSI for real]
Signed-off-by: Tom Rini <trini@konsulko.com>