meta-facebook: revamp OVERRIDES

Ensure OVERRIDES are specific, MACHINE or DISTRO, rather than extending
the primary OVERRIDES variable.  Fix the order so that the variables are
ordered from least-specific to most-specific since bitbake applies
overrides in increasing order.  This makes it so that 'fb-withhost' is
applied before 'fb-compute-multihost' is applied before 'yosemitev2'.
Thus, if a recipe has a specialization for 'yosemitev2' it is used
rather than the weaker specialization for 'fb-withhost'.

Tested by checking the OVERRIDES and MACHINEOVERRIDES for Bletchley,
Tiogapass, and Yosemitev2 to verify they look correct.

```
MACHINEOVERRIDES="armv7a:aspeed:aspeed-g6:fb-nohost:bletchley"
MACHINEOVERRIDES="armv6:aspeed:aspeed-g5:fb-withhost:fb-compute:fb-compute-singlehost:tiogapass"
MACHINEOVERRIDES="armv6:aspeed:aspeed-g5:fb-withhost:fb-compute:fb-compute-multihost:yosemitev2"
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Id5fa38de29c748e4ceafd5ef5c831b4f6434ea01
diff --git a/meta-facebook/conf/machine/include/facebook.inc b/meta-facebook/conf/machine/include/facebook.inc
index 43a0c22..4951a60 100644
--- a/meta-facebook/conf/machine/include/facebook.inc
+++ b/meta-facebook/conf/machine/include/facebook.inc
@@ -1,3 +1,5 @@
+DISTROOVERRIDES .= ":facebook"
+
 MACHINE_FEATURES += "\
         obmc-bmc-state-mgmt \
         obmc-phosphor-fan-mgmt \
@@ -14,4 +16,3 @@
 
 IMAGE_FEATURES:remove = "obmc-ikvm"
 
-OVERRIDES .= ":facebook"