Cosmetic changes to printed output, mostly related to probing devices...

 - Display slice and partition as <auto> instead of 0 or -1 when they're
   not set to specific values (the paritition=-1 was confusing folks).
 - When loaderdev isn't set in the u-boot environment, say so rather
   than displaying unknown device ''.
 - Print the loader(8) ident/version info earlier, so that all device-
   related info appears together afterwards.

The one change here that isn't purely cosmetic is to call setheap()
earlier.  The comment says "Initialise heap as early as possible", now
that's more accurate.  It shouldn't make any functional difference, but
may be safer if future changes lead to trying to allocate memory earlier.