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