[legacy] Allocate legacy driver .bss-like segments at probe time

[legacy] Allocate legacy driver .bss-like segments at probe time

Some legacy drivers use large static allocations for transmit and
receive buffers.  To avoid bloating the .bss segment, we currently
implement these as a single common symbol named "_shared_bss" (which
is permissible since only one legacy driver may be active at any one
time).

Switch to dynamic allocation of these .bss-like segments, to avoid the
requirement for using common symbols.

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