This updates three aspects of debugger/exception interactions:
- Save the user's "vector_catch" setting, and restore it after reset.
Previously, it was obliterated (rather annoyingly) each time.
- Don't catch BusFault and HardFault exceptions unless the user says
to do so. Target firmware may need to handle them.
- Don't modify SHCSR to prevent escalating BusFault to HardFault.
Target firmware may expect to handle it as a HardFault.
Those simplifications fix several bugs. In one annoying case, OpenOCD
would cause the target to lock up on ome faults which triggered after
the debugger disconnected.
NOTE: a known remaining issue is that OpenOCD can still leave DEMCR
set after an otherwise-clean OpenOCD shutdown.
Signed-off-by: David Brownell <firstname.lastname@example.org>