core: introduce the BR2_EXTERNAL variable

This commit introduces the BR2_EXTERNAL environment variable, which
will allow to keep Buildroot customization (board-specific
configuration files or root filesystem overlays, package Config.in and
makefiles, as well as defconfigs) outside of the Buildroot tree.

This commit only introduces the variable itself, and ensures that it
is available within Config.in options. This allows us to use
$BR2_EXTERNAL in a 'source' statement in Config.in.

Following patches extend the usage of BR2_EXTERNAL to other areas
(packages and defconfigs).

In details, this commit:

 * Introduces the BR2_EXTERNAL Kconfig option. This option has no
   prompt, and is therefore not visible to the user and also not
   stored in the .config file. It is automatically set to the value of
   the BR2_EXTERNAL environment variable. The only purpose of this
   BR2_EXTERNAL Kconfig option is to allow $BR2_EXTERNAL to be
   properly expanded when used inside Kconfig source statements.

 * Calculates the BR2_EXTERNAL value to use. If passed on the command
   line, then this value is taken in priority, and saved to a
   .br-external hidden file in the output directory. If not passed on
   the command line, then we read the .br-external file from the
   output directory. This allows the user to not pass the BR2_EXTERNAL
   value at each make invocation. If no BR2_EXTERNAL value is passed,
   we define it to support/dummy-external, so that the kconfig code
   finds an existing $(BR2_EXTERNAL)/package/Config.in file to
   include.

 * Passes the BR2_EXTERNAL into the *config environment, so that its
   value is found when parsing/evaluating Config.in files and .config
   values.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Tested-by: "Samuel Martin" <s.martin49@gmail.com>
Acked-by: "Samuel Martin" <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>