Added internal pull-up to PWR_BR.

o Prevent probe from inadvertently powering target.  If PWR_BR is
  allowed to float, the gate on Q1 (blackmagic_mini) will tend to be
  close enough to zero to turn the transistor on.  We activate the
  internal pull-up on this IO pin to force the transistor off.
Bugfix: Always apply the vector table magic number correctly.

Previously when the flash write length was less than the Chunk size the 2nd chunk write would re-calculate and apply an incorrect magic number
Add support for the SYS_WRITEC semihosting operation.

This is implemented in the same manner as QEMU semihosting
support, specifically, a SYS_WRITEC results in a single
character write to STDERR.
Configure SRST output as open-drain for BMP Mini hardware