disk: get_device_and_partition() "auto" partition and cleanup

Rework get_device_and_partition() to:
a) Implement a new partition ID of "auto", which requests that U-Boot
   search for the first "bootable" partition, and fall back to the first
   valid partition if none is found. This way, users don't need to
   specify an explicit partition in their commands.
b) Make use of get_device().
c) Add parameter to indicate whether returning a whole device is
   acceptable, or whether a partition is mandatory.
d) Make error-checking of the user's device-/partition-specification
   more complete. In particular, if strtoul() doesn't convert all
   characters, it's an error rather than just ignored.

The resultant device/partition returned by the function will be as
follows, based on whether the disk has a partition table (ptable) or not,
and whether the calling command allows the whole device to be returned
or not.

(D and P are integers, P >= 1)

D
D:
  No ptable:
    !allow_whole_dev: error
    allow_whole_dev: device D
  ptable:
    device D partition 1
D:0
  !allow_whole_dev: error
  allow_whole_dev: device D
D:P
  No ptable: error
  ptable: device D partition P
D:auto
  No ptable:
    !allow_whole_dev: error
    allow_whole_dev: device D
  ptable:
    first partition in device D with bootable flag set.
    If none, first valid paratition in device D.

Note: In order to review this patch, it's probably easiest to simply
look at the file contents post-application, rather than reading the
patch itself.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
[swarren: Rob implemented scanning for bootable partitions. I fixed a
couple of issues there, switched the syntax to ":auto", added the
error-checking rework, and ":0" syntax for the whole device]
Signed-off-by: Stephen Warren <swarren@nvidia.com>