classes/externalsrc: fix source being wiped out on clean with kernel

kernel.bbclass adds ${S} do do_clean[cleandirs], but this means if you
run bitbake -c clean <kernelrecipe> then your external source tree will
be trashed, which could be a disaster. For safety, remove ${S} from
cleandirs for every task. We also have to do the same for ${B} in the
case where EXTERNALSRC_BUILD is set to the same value as EXTERNALSRC.

(From OE-Core rev: b5071fb2667b8751885d38ca62fa36c870177cd5)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/externalsrc.bbclass

@@ -47,6 +47,20 @@ python () {
                 # Since configure will likely touch ${S}, ensure only we lock so one task has access at a time
                 d.appendVarFlag(task, "lockfiles", "${S}/singletask.lock")
 
+            # We do not want our source to be wiped out, ever (kernel.bbclass does this for do_clean)
+            cleandirs = d.getVarFlag(task, 'cleandirs', False)
+            if cleandirs:
+                cleandirs = cleandirs.split()
+                setvalue = False
+                if '${S}' in cleandirs:
+                    cleandirs.remove('${S}')
+                    setvalue = True
+                if externalsrcbuild == externalsrc and '${B}' in cleandirs:
+                    cleandirs.remove('${B}')
+                    setvalue = True
+                if setvalue:
+                    d.setVarFlag(task, 'cleandirs', ' '.join(cleandirs))
+
         for task in d.getVar("SRCTREECOVEREDTASKS", True).split():
             bb.build.deltask(task, d)