sdl: fix autoreconf timing issue

Context:
The autobuilders were failing on the symbol _XData32 being in conflicts.
A patch had been added to SDL to add a check to the configure.in

Problem:
Sometimes, the build would fail, because of an _XData32 symbol being in
conflicts eventhrough the patch was here.

What was happening:
Following the classic buildroot workflow:
   - Extract
   - [...]
   - Apply 001 patch, which touches configure.in AND configure
   - Apply 002 patch, which touches configure.in
   - Invoke autogen.sh
   - [...]

Right before running autogen.sh, we have configure.in which is more
recent than configure, which is fine.
We then, execute autogen.sh which, basically, runs autoconf.

If your machine was lighty loaded, the time difference between
configure.in and configure was really tiny (ms order), which seems to be
neglected by autoconf.
The results was that the configure was *NOT* generated. And our second
patch was taken into account.

If your machine was under heavy load, the time difference between the
two files would have been greater and then *maybe* picked up by
autoconf. And then the configure file was re-generated.

When the 0001 patch was introduced, SDL package did *NOT* run it's
autogen.sh, which is why it touches also the configure.
This came later, causing this behavior.

Fixes:
  http://autobuild.buildroot.net/results/d1c/d1c36f634dbf6b6e5d18444c2a23dfd129202b80/

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
avahi: Add systemd init script

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
uboot-tools: Allow users to use uboot's sources

If the user has specified a custom U-Boot repository, he may also want
to use it for U-Boot tools.

This could be usefull in two identified use case:
  - User want the same version for U-Boot tools and U-Boot
  - User has modified U-Boot tools in his U-Boot repository

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
---
Changes v1 -> v2:
    - Add a choice option (Suggested by Luca Ceresoli)
    - Add an if/then/else structure (Suggested by Luca Ceresoli)
    - Take UBOOT_LICENSE and UBOOT_LICENCE_FILES into account (Suggested
      by Arnout Vandecappelle)
infra: Add PRE_*_HOOKS for every step

Add PRE_*_HOOKS to all the different steps through which a package may go.

This will help avoid using POST_*_HOOKS to do tasks that should be done
in the PRE_*_HOOKS of the next step.
Otherwise, when the user would do a make foo-re<step>, this would not do
what was really intented, the POST_*_HOOK of the preceding step not
being executed.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
libenca: Move build host tool to PRE_BUILD_HOOKS

The make_hash host tool, should be built during a PRE_BUILD_HOOKS and
not a POST_CONFIGURE_HOOKS.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>