Makefile.toolchain.zephyr: Modifications for SDK 0.9

Various changes required by new SDK 0.9:

1. Renamed toolchain vendor name ("poky" -> "zephyr")
2. Toolchains now 64 bit, previously 32 bit
3. New toolchain for Xtensa
4. New toolchain for RISC-V

0.8.2 is still supported.

Change-Id: Icfd4cc44ef643da0f3840b33d6dfc4b3c99dfa9a
Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>