packageJopenjdk: add support for building the full jdk

packageJopenjdk: add support for building the full jdk

Some users may require the full JDK on the target to debug their programs.
This change is relatively trivial to add.

While the full JDK does have programs used for compiling on a target,
which is against Buildroot policy, the JDK also has several utilities used for
debugging purposes, which the JRE target does not build, and Buildroot supports
applications used for debugging purposes such as GDB.

As such, JDK support should be available for debugging purposes, and a note in
the Config.in file has been added under the JDK section, which informs the user
that JDK support is for debugging purposes only and that developing on a
target is not supported by Buildroot.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Reviewed-by: Ryan Barnett <ryan.barnett@rockwellcollins.com>
Tested-by: Ryan Barnett <ryan.barnett@rockwellcollins.com>
[yann.morin.1998@free.fr:
  - s/OPENJDK_INSTALL_DIR/OPENJDK_VARIANT/
  - slightly rewrap help text
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>