helpers/*: use '.' not 'source'

The 'source' command is a bashism, and not in the POSIX standard. So
it fails when using POSIX compliant shell such as Dash which is the
default on Debian. Using the '.' command is equivalent to using
source, and is POSIX compliant.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>

helpers/assert_device_present

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

helpers/assert_driver_present

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

helpers/ensure_lib_firmware

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