Support "bootrom" as config option + include generic example

Factor out the creation of the bootram
+ cleaned much boiler plate in the config parser.

RISCVEMU originally included a short, but hard-coded bootrom.  That
evolved organically to an ever more complicated bootrom generation,
which was finally supplemented by the now perfered option of loading a
custom bootrom.

This change include a small example rom, and reorganizes the bootrom
handling code slightly.