meta-facebook: add secondary-emmc feature

  * Define a MACHINE_FEATURE that enables a secondary eMMC device.
  * Add kernel and base-files support for mounting the eMMC device
    using btrfs.
  * Enable machine feature for Bletchley.

Signed-off-by: Potin Lai <potin.lai@quantatw.com>
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I3fde3b285720c137c4e5bbbad233e4ef40b84d54
diff --git a/meta-facebook/conf/machine/include/facebook-secondary-emmc.inc b/meta-facebook/conf/machine/include/facebook-secondary-emmc.inc
new file mode 100644
index 0000000..2a63e97
--- /dev/null
+++ b/meta-facebook/conf/machine/include/facebook-secondary-emmc.inc
@@ -0,0 +1,2 @@
+MACHINE_FEATURES:append = " fb-secondary-emmc"
+MACHINEOVERRIDES:append = ":mf-fb-secondary-emmc"