dist/tools/buildsystem_sanity_check: make shellcheck happy

The build system sanity check test script currently does not pass
shellcheck, preventing PRs improving it from getting merged.

This commit fixes the issues pointed out by shellcheck and should not
have any functional changes.

dist/tools/buildsystem_sanity_check/check.sh

@@ -16,14 +16,14 @@
 : "${RIOTBASE:="$(cd "$(dirname "$0")/../../../" || exit; pwd)"}"
 
 : "${RIOTTOOLS:=${RIOTBASE}/dist/tools}"
+# shellcheck source=../ci/github_annotate.sh
 . "${RIOTTOOLS}"/ci/github_annotate.sh
 
 SCRIPT_PATH=dist/tools/buildsystem_sanity_check/check.sh
 
 
 tab_indent() {
-    # Ident using 'bashism' to to the tab compatible with 'bsd-sed'
-    sed 's/^/\'$'\t/'
+    sed 's/^/\t/'
 }
 
 prepend() {
@@ -34,13 +34,13 @@ prepend() {
 }
 
 error_with_message() {
-    while read INPUT; do
+    while read -r INPUT; do
         if github_annotate_is_on; then
             MESSAGE="${1}"
             FILE=$(echo "${INPUT}" | cut -d: -f1)
             LINE=$(echo "${INPUT}" | cut -d: -f2)
             MATCH=$(echo "${INPUT}" | cut -d: -f3)
-            if [[ $var =~ ^[0-9]+$ ]] || [  -z "$MATCH" ]; then
+            if [[ $LINE =~ ^[0-9]+$ ]] || [  -z "$MATCH" ]; then
                 # line is not provided in grep pattern
                 LINE=0
             fi
@@ -172,7 +172,7 @@ check_not_exporting_variables() {
     # Only run if there are patterns, otherwise it matches everything
     if [ ${#patterns[@]} -ne 0 ]; then
         git -C "${RIOTBASE}" grep -n "${patterns[@]}" -- "${pathspec[@]}" \
-            | error_with_message 'Variables must only be exported in `makefiles/vars.inc.mk`'
+            | error_with_message "Variables must only be exported in \`makefiles/vars.inc.mk\`"
     fi
 }
 
@@ -287,14 +287,14 @@ check_files_in_boards_not_reference_board_var() {
     local patterns=()
     local pathspec=()
 
-    patterns+=(-e '/$(BOARD)/')
+    patterns+=(-e "/\$(BOARD)/")
 
     pathspec+=('boards/')
     # boards/common/nrf52 uses a hack to resolve dependencies early
     pathspec+=(':!boards/common/nrf52/Makefile.include')
 
     git -C "${RIOTBASE}" grep -n "${patterns[@]}" -- "${pathspec[@]}" \
-        | error_with_message 'Code in boards/ should not use $(BOARDS) to reference files since this breaks external BOARDS changing BOARDSDIR"'
+        | error_with_message "Code in boards/ should not use \$(BOARDS) to reference files since this breaks external BOARDS changing BOARDSDIR"
 }
 
 check_no_pseudomodules_in_makefile_dep() {
@@ -415,7 +415,7 @@ all_checks() {
 
 main() {
     all_checks | prepend 'Invalid build system patterns found by '"${0}:" | error_on_input >&2
-    exit $?
+    return $?
 }