Travis-CI: Switch back to using the top of tree dtc

In a0f3e3df4adc we switched to using the Ubuntu-provided dtc as travis
was having a problem with the number of warnings that were generated by
the newer dtc.  This is no longer a concern as we now have the same
logic as Linux to enable/disable additional more stringent warnings.  Go
back to building dtc from source.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested on travis-ci:
Tested-by: Simon Glass <sjg@chromium.org>

.travis.yml

@@ -26,12 +26,11 @@ addons:
     - grub-efi-ia32-bin
     - rpm2cpio
     - wget
-    - device-tree-compiler
 
 install:
  # install latest device tree compiler
- #- git clone --depth=1 git://git.kernel.org/pub/scm/utils/dtc/dtc.git /tmp/dtc
- #- make -j4 -C /tmp/dtc
+ - git clone --depth=1 git://git.kernel.org/pub/scm/utils/dtc/dtc.git /tmp/dtc
+ - make -j4 -C /tmp/dtc
  # Clone uboot-test-hooks
  - git clone --depth=1 git://github.com/swarren/uboot-test-hooks.git /tmp/uboot-test-hooks
  - ln -s travis-ci /tmp/uboot-test-hooks/bin/`hostname`