Force OLDEST_KERNEL to 4.4.0

This is the oldest kernel currently used by openbmc, so there is no
point in compiling glibc support older kernels.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/conf/distro/openbmc-phosphor.conf b/conf/distro/openbmc-phosphor.conf
index f37e88c..b4e6f7a 100644
--- a/conf/distro/openbmc-phosphor.conf
+++ b/conf/distro/openbmc-phosphor.conf
@@ -5,6 +5,7 @@
 DISTRO_VERSION = "0.1.0"
 TARGET_VENDOR="-openbmc"
 
+OLDEST_KERNEL = "4.4.0"
 GCCVERSION ?= "4.9.3"
 IMAGE_FSTYPES += "cpio.gz"
 IMAGE_FSTYPES += "squashfs-xz"