tests/bsim: All scripts: Use boardtarget string instead of BOARD

In all scripts, where we were using the BOARD variable
let's use BOARD_TS which is the full BOARD target string
with "/" replaced with "_"
This is neccessary to support hwmv2 board names

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>