Fix typo in MVPD_WRITE_TO_PNOR

See https://github.com/open-power/op-build/issues/483
There is a typo in the config where FROM was used instead of TO.
It turns out that it doesn't functionally matter due to some
default rules we have, but it is very confusing.
diff --git a/openpower/configs/hostboot/firestone.config b/openpower/configs/hostboot/firestone.config
index e8218cf..4dd3cde 100755
--- a/openpower/configs/hostboot/firestone.config
+++ b/openpower/configs/hostboot/firestone.config
@@ -9,7 +9,7 @@
 set MVPD_READ_FROM_HW
 set MVPD_WRITE_TO_HW
 set MVPD_READ_FROM_PNOR
-set MVPD_WRITE_FROM_PNOR
+set MVPD_WRITE_TO_PNOR
 set DJVPD_READ_FROM_HW
 set DJVPD_WRITE_TO_HW
 set DJVPD_READ_FROM_PNOR