cortex-m: step over breakpoint

[flit]

Implement step over breakpoint for the CortexM class.

This patch handles single and range step over both software breakpoints and bkpt instructions in the original code. Resume will also automatically skip over a bkpt instruction, but does not handle software breakpoints. This is because there is no event available outside the gdbserver that can be used to reinstall a temporarily-removed software breakpoint when the core halts.

The basic_test.py functional test is extended to cover all step over breakpoint combinations.

Fixes #1537