Reset vector should be a config file parameter

For this and all other numeric options, if given as a string, we allow hexadecimal constants.

Building a bootrom and/or DTB should be a option (disabled by default?)

Start by locating how configuration and numeric options are parsed, then trace the reset vector handling and the bootrom/DTB build flow. Clarify whether hexadecimal strings should work for every numeric option and whether bootrom and DTB generation should be optional and disabled by default. Done means the configuration behavior and build options are specified and covered by appropriate tests.

Written by the indexing model from the issue text.