In jtagboot and flashboot, put .bss afer .data, as we're not filling the
memory and directly loading the ELF binary will otherwise cause global
variables to point to the wrong bits of memory.
Clean up generated ELF files that the BSD make infrastructure doesn't know
about.

Don't try to use the generic linker script instead of the target-specific
script.
Remove now-unused generic linker script for boot2.
Enable slightly more boot2 argument parsing: allow -P to be used.