The helpers assume that bootrr script is located in /usr/bin, which is
not always the case, especially when one needs to run the test locally
from the git tree.
'source' command will search for files using $PATH. When bootrr
scripts are installed globally using 'make install' they will be
found. When running them from a local folder, setting PATH to include
<bootrr>/helpers will ensure that all scripts will be found.
Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>