Hi,

This patch adds code to the elf loader which allows it to classify input 
sections as code or not-code. It also adds a passthrough bus to the 
loader ("probe-upstream", "probe-downstream"). When the "probe-upstream" 
bus is accessed, the loader checks the address against known code 
sections and drives the "write-to-code_address" pin if a write to a code 
section is attempted. All accesses are passed on to "probe-downstream" 
accessor.

This is useful in generating simulation time warnings for writes to code 
sections.

ok to commit?

Dave