Target hooks

[AndrewD]

This allows the target to supply custom target_init() and target_boot() functions, via a library linked with the bios. eg:

    # add custom target specific library to the bios:
    src="libtarget"
    src_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), src))
    builder.add_software_package(src, src_dir)
    builder.add_software_library(src)