Build: Adds DISCARD_PACKAGES variable in build scripts

Updates build script according recent changes in efibuild. We need to
specify OpenCorePkg to be removed while build because currently this 
package is present as submodule in audk and conflicts

build_duet.tool

@@ -154,10 +154,13 @@ else
     ARCHS=(X64 IA32)
     export ARCHS
   fi
+
+  DISCARD_PACKAGES=OpenCorePkg
   SELFPKG_DIR="OpenCorePkg"
   SELFPKG=OpenDuetPkg
   NO_ARCHIVES=1
 
+  export DISCARD_PACKAGES
   export SELFPKG_DIR
   export SELFPKG
   export NO_ARCHIVES

build_oc.tool

@@ -292,9 +292,11 @@ if [ "$ARCHS" = "" ]; then
 fi
 SELFPKG=OpenCorePkg
 NO_ARCHIVES=0
+DISCARD_PACKAGES=OpenCorePkg
 
 export SELFPKG
 export NO_ARCHIVES
+export DISCARD_PACKAGES
 
 src=$(curl -Lfs https://raw.githubusercontent.com/acidanthera/ocbuild/master/efibuild.sh) && eval "$src" || exit 1