build: interpose zephyr_flash_debug.py in flash target

Zephyr board flashing and debugging is done via shell scripts. It
would improve the CMake transition to remove the shell dependency.

Add zephyr_flash_debug.py to allow phasing out the shell scripts.
This takes two arguments:

- a command (eventually flash, debug, and debugserver, but just flash
  for now)

- the path to the corresponding shell script

zephyr_flash_debug.py runs the command in pure Python if it
knows how. Otherwise, it falls back on the shell script. In
this patch, it always falls back. Subsequent patches add support
for existing flash backends.

Invoke zephyr_flash_debug.py from the Makefile flash target, but only
if USE_ZEPHYR_FLASH_DEBUG_SHELL is empty. This lets users keep existing
behavior in case of issues, and can be removed later once the Python
script is more widely tested.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>