Update loader's linker-script comments to reflect the fact that we no longer
plan to have loader self-relocate.
Make room for ELF headers in memory layout for loader, since we will use th
ELF file rather than a  e ory image.
Remove thoroughly out-of-date comment in loader's startup assembly: we are not
boot2 and we don't use its argument register conventions.
Explicitly zero BSS for /boot/loader since we now use ELF not a memory
image.  Consistently name BSS and heap mempry symbols.
Start reworking boot2 for a less x86 world order: no ATA, SCSI, floppy disks:
instead CFI flash, SD cards, and JTAG direct load.  Default to CFI without
partition tables; introduce a boot path that supports a JTAG-loaded second-
stage loader at 0x20000; a mini-SD card boot driver is not yet present.

With these changes, boot2 on cheri can now run the second-stage loader if it
has been conveniently plopped into DRAM, including the forth interpreter.  I
have not yet tried loading a kernel with it, nor exercised the CFI case,
which might (or might not) just work.