build: reports: add puncover target

puncover is a footprint and stack analysis tool in python, see

https://github.com/HBehrens/puncover



Add custom target puncover which launches puncover with the right
options.

Launch for example with:

west  build -b frdm_k64f samples/hello_world/ -t puncover

Signed-off-by: Anas Nashif <anas.nashif@intel.com>