Thanks are due to Marcus Roeder, who pointed out an error in my DreamPlug kernel compile workflow (http://rglinuxtech.com/?p=1354)..

The make LOADADDR.. command was incorrect, and would result in less system memory being available..

The correct command is:

make LOADADDR=000008000 uImage

– and this will result in all the memory being available:

# free
      total  used   free  shared buffers cached
Mem: 512648 143796 368852 0      18968    74800
.............

I have amended the original article..  Apologies for the confusion..

Robert Gadsdon.  March 29, 2015.