contribs: bootstrap: allow config.mak override file

The file is optional but is still used by ports and needed to avoid
using global environment configuration variables in build scripts.

(cherry picked from commit 9234532d61)
Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>

contrib/bootstrap

@@ -403,6 +403,7 @@ TOPSRC_BUILT=$(python3 -c "import os; print(os.path.relpath('$BOOTSTRAP_PATH', '
 add_make "TOPSRC = $TOPSRC"
 add_make "TOPSRC_BUILT = $TOPSRC_BUILT"
 add_make "TOPDST = .."
+add_make "-include config.mak"
 add_make 'include $(TOPSRC)/src/main.mak'
 echo "Bootstrap completed."
 make help