Update kernel and initramfs build distro

They were hardcoded to EOL'd Ubuntu versions. Use 'latest' instead.

Change-Id: I1ef8e9be38066f046b5741799006f6b847664241
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/initramfs-build.sh b/initramfs-build.sh
index 4a9ad7d..bbbb67b 100755
--- a/initramfs-build.sh
+++ b/initramfs-build.sh
@@ -19,7 +19,7 @@
 fi
 
 Dockerfile=$(cat << EOF
-FROM ubuntu:15.10
+FROM ubuntu:latest
 
 ${PROXY}
 
diff --git a/kernel-build.sh b/kernel-build.sh
index 21dd093..83f0d56 100755
--- a/kernel-build.sh
+++ b/kernel-build.sh
@@ -19,7 +19,7 @@
 fi
 
 Dockerfile=$(cat << EOF
-FROM ubuntu:15.10
+FROM ubuntu:latest
 
 ${PROXY}