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 <alpi@oticon.com>