tests: bsim_bt: Add option to re-build incrementally
tests: bsim_bt: Add option to re-build incrementally

In the provided compile.sh script (which builds all bsim_bt
testcases used in the CI regression runs) add the option to
do an incremental build (if possible).
This is just a nicety for users who want to use this script
while testing locally.

This is controller with the existance of the variable INCR_BUILD

For ex., the script can be called, as:
 WORK_DIR=${ZEPHYR_BASE}/my_work_folder INCR_BUILD=y \
 tests/bluetooth/bsim_bt/compile.sh

Signed-off-by: Alberto Escolar Piedras <52be3174920ed06cbdfbcf9622a1a6c68e942617@oticon.com>

explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,nashif/zephyr,punitvara/zephyr,ldts/zephyr,galak/zephyr,punitvara/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,Vudentz/zephyr,ldts/zephyr