[IOT-2029] split scons builder code out of external_libs.scons

[IOT-2029] split scons builder code out of external_libs.scons

This splits the contents of the external_libs.scons into two
files:
   external_builders.scons
   external_libs.scons

The external_libs.scons script added scons  Psuedo-Builders
'PrepareLib', 'Download', 'UnpackAll', 'Configure',
'InstallHeadFile' and 'InstalLib' to the scons environment
The builder scripts are responsible for downloading, unpacking,
and installing different items throughout the build process.

Additionally the external_libs.scons was also responsible for
building and setting up compilar flags and building external
libraries.

The external_libs.scons used to be called before and after the
android build. Once to setup the Psuedo-Builders and again to
set the compiler flags for the external libraries. Due to some
cleanup for a recent change. The script can no longer be called
multiple times without a scons error.

see: https://gerrit.iotivity.org/gerrit/#/c/18381

Now the Psuedo-Builders are added to the scons environment by
calling external_builders.scons. The external_libs.scons is
used to build the external libraries.

This allows adding the builders that are required for the
android build. While still setting up the build environment
for the external libraries.

Due to the UnpackAll builder using the system path on windows
the external_builders.scons script must be called twice.

Additional Whitespace cleanup changed tabs into spaces to follow
convention of other scons scripts.

Change-Id: I070224425bfd2d4279cd6c9b0c329e393250770a
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18739
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>
Tested-by: Uze Choi <uzchoi@samsung.com>