externalsrc: do not call make clean for recipes with CLEANBROKEN = "1" set

externalsrc: do not call make clean for recipes with CLEANBROKEN = "1" set

ERROR: distrho-ports-0.0.0+git999-r0 do_buildclean: oe_runmake failed
ERROR: distrho-ports-0.0.0+git999-r0 do_buildclean: Function failed: do_buildclean
ERROR: Logfile of failure stored in: <...>/temp/log.do_buildclean.17285
Log data follows:
| DEBUG: Executing shell function do_buildclean
| NOTE: make clean
| make clean -C libs/drowaudio
| make[1]: Entering directory '/home/a.mueller/data/oe-core/workspace/sources/distrho-ports/libs/drowaudio'
| make clean -C build-drowaudio
| make[2]: Entering directory '/home/a.mueller/data/oe-core/workspace/sources/distrho-ports/libs/drowaudio/build-drowaudio'
| make[2]: *** No rule to make target 'clean'.  Stop.
| make[2]: Leaving directory '/home/a.mueller/data/oe-core/workspace/sources/distrho-ports/libs/drowaudio/build-drowaudio'
| make[1]: *** [Makefile:7: clean] Error 2
| make[1]: Leaving directory '/home/a.mueller/data/oe-core/workspace/sources/distrho-ports/libs/drowaudio'
| make: *** [Makefile:73: clean] Error 2
| ERROR: oe_runmake failed

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>