helpers/*: do not assume /usr/bin/

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>

helpers/assert_device_present

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-source /usr/bin/bootrr
+source bootrr
 
 TEST_CASE_ID="$1"
 DRIVER="$2"

helpers/assert_driver_present

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-source /usr/bin/bootrr
+source bootrr
 
 TEST_CASE_ID="$1"
 DRIVER="$2"

helpers/ensure_lib_firmware

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-source /usr/bin/bootrr
+source bootrr
 
 TEST_CASE_ID="$1"
 PARTITION=/dev/disk/by-partlabel/userdata