murdock: add prefix support to get_app_board_toolchain_pairs()

Previously, get_app_board_toolchain_pairs() took the appdir as only
parameter and produced "$appdir $board:$toolchain" tuples.
This commit makes get_app_board_toolchain_pairs() prefix each tuple with
any additional args, e.g.,

    get_app_board_toolchain_pairs() tests/hello_world ./murdock compile

->

    ./murdock compile tests/hello_world samr21-xpro:gnu

This allows getting rid of the xargs call in get_compile_jobs().

.murdock

@@ -118,10 +118,15 @@ get_supported_toolchains() {
 
 # given an app dir as parameter, print "$appdir $board:$toolchain" for each
 # supported board and toolchain. Only print for boards in $BOARDS.
+# if extra args are given, they will be prepended to each output line.
 get_app_board_toolchain_pairs() {
     local appdir=$1
     local boards="$(get_supported_boards $appdir)"
 
+    # collect extra arguments into prefix variable
+    shift
+    local prefix="$*"
+
     if [ "$boards" = makefile_broken ]; then
         echo "$appdir makefile_broken"
         return
@@ -131,7 +136,7 @@ get_app_board_toolchain_pairs() {
     do
         for toolchain in $(get_supported_toolchains $appdir $board)
         do
-            echo $appdir $board:$toolchain
+            echo $prefix $appdir $board:$toolchain
         done
     done | $(_greplist $BOARDS)
 }
@@ -140,8 +145,7 @@ get_app_board_toolchain_pairs() {
 get_compile_jobs() {
     get_apps | \
         dwqc ${DWQ_ENV} -s \
-        "$0 get_app_board_toolchain_pairs \${1}" \
-        | xargs '-d\n' -n 1 echo $0 compile
+        "$0 get_app_board_toolchain_pairs \${1} $0 compile"
 }
 
 print_worker() {