For omap platforms: Make sure boot_params_ptr is set in both SPL and
non-SPL builds, and make a copy of boot params passed by SPL to the
non-SPL next stage in that stage, including the boot device type.
For am33xx boards, if the boot device was an MMC device, automatically
set the mmcdev environment variable to that device so the next load
will naturally happen from the boot device.
- Correctly set the more flag in dev_stor_get() when first is true.

- Report present-but-unknown devices in the dev_stor_get() results, so
  things like empty mmc card slots don't stop the enumeration process.
  The rest of the device/storage API already sanely handles such
  unknown devices.
- Report non-responsive mmc slots as unknown devices of zero size.

- Change some messages to debug-only to reduce the spew that results
  from the very repetitively-probing device enumeration process.