Fix shellcheck issues (#1341)

* Fix issue with latest shellcheck version

The latest shellcheck versions use a new error number for non-POSIX
string replacement. Change to ignore this new error number.

* Ignore shellcheck issue about not following sourced files

Newer shellcheck versions also warn when shellcheck does not follow
sourcing of files with known path:
  Not following: ./meta was not specified as input (see shellcheck -x).

We check those files separately so ignore this error for the two scripts
affected.

buildroot-external/scripts/post-build.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# shellcheck disable=SC1090
+# shellcheck disable=SC1090,SC1091
 set -e
 
 SCRIPT_DIR=${BR2_EXTERNAL_HASSOS_PATH}/scripts

buildroot-external/scripts/post-image.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# shellcheck disable=SC1090
+# shellcheck disable=SC1090,SC1091
 set -e
 
 SCRIPT_DIR=${BR2_EXTERNAL_HASSOS_PATH}/scripts