Fix mtd partitions udev rule

Prevent the ro partitions from triggering the udev rule. They don't have the
necessary "name" attribute and so udev attempts to symlink them to the base
/dev/mtd/ directory.

Resolves openbmc/openbmc#2172

Change-Id: I438feb796b3e5c070949757d178d5382ce3a9e0f
Signed-off-by: Eddie James <eajames@us.ibm.com>
diff --git a/common/recipes-bsp/u-boot/files/76-mtd-partitions.rules b/common/recipes-bsp/u-boot/files/76-mtd-partitions.rules
index 16a7cfe..28e4586 100644
--- a/common/recipes-bsp/u-boot/files/76-mtd-partitions.rules
+++ b/common/recipes-bsp/u-boot/files/76-mtd-partitions.rules
@@ -1 +1 @@
-SUBSYSTEM=="mtd", SYMLINK+="mtd/%s{name}"
+ENV{DEVTYPE}=="mtd", SYMLINK+="mtd/%s{name}"