sdk: add support for Zephyr SDK 0.11

This adds support for Zephyr SDK 0.11.* is based on the 0.10 support
with the following changes:

Handle how xtensa toolchains for given targets are organized in SDK
0.11.x.  They have their own directories per specific xtensa SoC.

The x86_64 toolchain in the SDK has been updated to build for 32-bit
targets (as it has 32-bit soft float libraries).  Use this toolchain for
all x86 builds.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>