[pcbios] Add extra debugging messages when unhiding iPXE from memory

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[nbiprefix] Set up real-mode stack before jumping to .text16

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[prefix] Use %cs as implicit parameter to uninstall()

romprefix.S currently calls uninstall() with an invalid value in %ax.
Consequently, base memory is not freed after a ROM boot attempt (or
after entering iPXE during POST).

The uninstall() function is physically present in .text16, and so can
use %cs to determine the .text16 segment address.  The .data16 segment
address is not required, since uninstall() is called only by code
paths which set up .data16 to immediately follow .text16.

Signed-off-by: Michael Brown <mcb30@ipxe.org>