sim: Basic ram-load test

Test the basic configuration for ram loading.  Instead of a fixed
address for RAM, the values come dynamically from a thread-local
variable (allowing the tests to run in parallel).  The size of the ram
along with the address of the buffer in the test address space are
passed in this way.

This tests the single-image configurations of ram loading.  Testing
multi-image will take additional work, as the RAM will need to be large
enough for both images, and the second image will need a meaningful
offset address in RAM.

Signed-off-by: David Brown <david.brown@linaro.org>