PowerPC: Update last users of CONFIG_ISO_STRING to Kconfig

There are a few boards that use CONFIG_ISO_STRING as part of a sanity
check during firmware update at run time.  Move this string to Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/mpl/pip405/Kconfig b/board/mpl/pip405/Kconfig
index f485367..e7ae6af 100644
--- a/board/mpl/pip405/Kconfig
+++ b/board/mpl/pip405/Kconfig
@@ -9,4 +9,7 @@
 config SYS_CONFIG_NAME
 	default "PIP405"
 
+config ISO_STRING
+	string
+	default "MEV-10066-001"
 endif