tests/bsim/sh_common.source: Set a variable with the board target string

As now, with hwmv2, boards can have "/" in their name,
and the board name is not anymore the complete target
name, let's set a new variable with the
whole board target name, but converted into the format
which can be used in paths.

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