fix another nonportable grep usage in scripts

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

scripts/mkconfigs

@@ -63,7 +63,7 @@ echo "\
  */
 static const char bbconfig_config_bz2[] ALIGN1 = {"
 
-grep '^#\? \?CONFIG_' "$config" \
+grep -e '^# CONFIG_' -e '^CONFIG_' "$config" \
 | bzip2 -1 | dd bs=2 skip=1 2>/dev/null \
 | od -v -t x1 \
 | sed -e 's/^[^ ]*//' \