debugger: staticize private functions.
rtl/memory: correctly compute byteselects.

We need to compute byte selects from the real address not the bus
address (which is always 32-bit aligned).
rtl/ram: enable byte selects for on-chip memory.
debugger: add a tool for loading elf files.

This means we can easily load elf without any kind of bootloader or
converting to binary files first.