travis-ci: turn on cache for cross-compiler

Added cross-compiler to $PATH in continuous-build.sh
and enabled sudo in .travis.yml.

NOTES:

- travis is unable to update submodules pointing to commits out of their master branch,
  so if you push to devtools, you need to push into master branch or your build will be broken
- travis caches conflicts with submodule managements, so we use tmp/toolchain instead of
  hacking/cross/toolchain in .travis.yml and continuous-build.sh will mv binaries as required.

.travis.yml

@@ -1,6 +1,17 @@
 language: c
-install:
-- git submodule init
+git:
+  submodules: false
+sudo: required
+dist: trusty
+cache:
+  timeout: 1200
+  directories:
+  - tmp/toolchain/
+before_install:
+- sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
+- sudo add-apt-repository ppa:ubuntu-lxc/lxd-stable -y
+- sudo apt-get update
+- sudo apt-get install -y --force-yes $CC rc golang build-essential bison qemu-system
 - git submodule update --init --recursive --remote
 script:
 - ./hacking/continuous-build.sh

hacking

@@ -1 +1 @@
-Subproject commit 8bcf5fe6de83941370260cf8315cd50e5c2e63d7
+Subproject commit dc7ca89d09380789418b98dee5ceef06a3a9120e