main: rework panic handler
When an OSMO_ASSERT() fails it will call the osmo panic handler.
Previous it will always calls into a breakpoint which will require
a power-cycle.
Change the default (for production) to use a system reset.
But still allow calling into breakpoint by setting `break_on_panic` via SWD.
Change-Id: I91c732001e848ec5509bf81af8aac1a2a80eb718
