cmake: targets: support overriding puncover host and port

Allow overriding puncover default host and port. Using
PUNCOVER_HOST/PUNCOVER_PORT it is possible now to set the host and port
to something else other than the default.

example:

export PUNCOVER_HOST=10.0.1.1
export PUNCOVER_PORT=8088

will start puncover on 10.0.1.1:8088 and make it available over the
local network.

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