imx233: fix partition window computation

The code was broken in two ways:
- it called storage_read_sectors with a wrong drive number
- calling storage_read_sectors too early at boot time will fail because
  we are in the init function, so the drive is not yet registered.
To fix this, use a user provided read callback instead of storage read
functions.

Change-Id: I5ab79d48391fae619d5f8ae09c3d499fc43854c8
3 files changed