externalsrc: Set STAMPCLEAN to match STAMP

Only the last stamp file should be kept, but unless STAMPCLEAN matches
files generated using STAMP old stamp files may linger.  This may
cause false positives for skipping tasks.

(From OE-Core rev: 6d60f60e38e0e00e6753f5b136277f27d6204e63)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/externalsrc.bbclass

@@ -116,6 +116,7 @@ python () {
             configstamp = '${TMPDIR}/work-shared/${PN}/${EXTENDPE}${PV}-${PR}/configure.sstate'
             d.setVar('CONFIGURESTAMPFILE', configstamp)
             d.setVar('STAMP', '${STAMPS_DIR}/work-shared/${PN}/${EXTENDPE}${PV}-${PR}')
+            d.setVar('STAMPCLEAN', '${STAMPS_DIR}/work-shared/${PN}/*-*')
 }
 
 python externalsrc_configure_prefunc() {