infra: Introduce BR2_HOST_NEEDS_JAVA

XBMC needs Java on the host in order to build, because it uses a
code-generator which is built in two phases: In the first phase SWIG is used
to parse C++ header files that define the API.  SWIG outputs an XML file
that contains a complete description of the structure of the API.  In the
second phase, the XML file is ingested by a Groovy (Java) program that then
creates C++ code that forms the bridge to the scripting language (Python).

The second phase is why we need java on the host.

You can learn more at the XBMC's wiki:
http://wiki.xbmc.org/index.php?title=Codegeneration#How_it_works



In order to check that, this patch introduce this mechanism in
dependencies.sh, and it also defines the variable in Config.in

[Peter: fix error message]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>