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)
package: Add PRE_*_HOOKS

Add PRE_*_HOOKS to all the different steps through which a package has to 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>