rm_work: handle non-existant stamps directory

Since 'build/siggen: Rework stamps functions'[1] the stamps directory
for each recipe is only created if there are stamps to be written. Some
recipes - such as package-index - only have nostamp tasks so the stamps
directory is never created, resulting in the rm_work task trying to cd
into a directory that doesn't exist.

Resolve this by simply checking the stamps directory exists before
trying to delete files in it.

[1] bitbake c79ecec580e4c2a141ae483ec0f6448f70593dcf

(From OE-Core rev: 80e9b32a000b25420997068543d3631231eb3ccf)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/rm_work.bbclass

@@ -57,7 +57,9 @@ do_rm_work () {
     # Change normal stamps into setscene stamps as they better reflect the
     # fact WORKDIR is now empty
     # Also leave noexec stamps since setscene stamps don't cover them
-    cd `dirname ${STAMP}`
+    STAMPDIR=`dirname ${STAMP}`
+    if test -d $STAMPDIR; then
+        cd $STAMPDIR
         for i in `basename ${STAMP}`*
         do
             case $i in
@@ -106,6 +108,7 @@ do_rm_work () {
                 "${RM_BIN}" -f -- $i
             esac
         done
+    fi
 
     cd ${WORKDIR}
     for dir in *