murdock: make get_compile_jobs() return subjobs

This change allows callers to receive the job list of individual apps in
chunks (instead of all at once after all appdirs have been processed),
if run as dwq job itself.

E.g.,

    $ dwqc ./.murdock get_compile_jobs

would previously run "get_compile_jobs()" as one job, collecting all the
output from "get_app_board_toolchain_pairs()" subjobs, then return the
combined output as job result.

By using subjobs, the job returns right away, but has previously sent
the "get_app_board_toolchain_pairs" jobs as subjobs ti the initial
instance, which will also wait for all of them to complete, but
already print subjob output as it is received.

.murdock

@@ -145,6 +145,7 @@ get_app_board_toolchain_pairs() {
 get_compile_jobs() {
     get_apps | \
         dwqc ${DWQ_ENV} -s \
+        ${DWQ_JOBID:+--subjob} \
         "$0 get_app_board_toolchain_pairs \${1} $0 compile"
 }