Support signed attributes with override tool

Change-Id: Ib82466e9a2b8da9c436707e463dd2669d59aedd7
RTC: 144142
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/40772
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins OP Build CI <op-jenkins+hostboot@us.ibm.com>
Tested-by: FSP CI Jenkins <fsp-CI-jenkins+hostboot@us.ibm.com>
Reviewed-by: Matthew A. Ploetz <maploetz@us.ibm.com>
Reviewed-by: Daniel M. Crowell <dcrowell@us.ibm.com>

Author: Dzuy Nguyen

src/usr/targeting/attroverride/attrTextToBinaryBlob.C

@@ -950,7 +950,9 @@ bool AttrTextToBinaryBlob::attrFileAttrLinesToData(
             }
         }
 
-        if ((l_valString[0] == '0') && (l_valString[1] == 'x'))
+        // Expect 0x<> for unsigned and -<> for signed attribute
+        if ((l_valString[0] == '0') && (l_valString[1] == 'x') ||
+           (l_valString[0] == '-'))
         {
             // Value string is a value
             l_attrVal = strtoull(l_valString.c_str(), NULL, 0);