[isa] Disable legacy ISA device probing by default

[isa] Disable legacy ISA device probing by default

Legacy ISA device probing involves poking at various I/O addresses to
guess whether or not a particular device is present.

Actual legacy ISA cards are essentially nonexistent by now, but the
probed I/O addresses have a habit of being reused for various
OEM-specific functions.  This can cause some very undesirable side
effects.  For example, probing for the "ne2k_isa" driver on an HP
Elitebook 840 G10 will cause the system to lock up in a way that
requires two cold reboots to recover.

Enable ISA_PROBE_ONLY in config/isa.h by default.  This limits ISA
probing to use only the addresses specified in ISA_PROBE_ADDRS, which
is empty by default, and so effectively disables ISA probing.  The
vanishingly small number of users who require ISA probing can simply
adjust this configuration in config/local/isa.h.

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