If the user hasn't set EXTERNAL_TOOLCHAIN, but CODEBENCH_PATH is set, then we
automatically set both EXTERNAL_TOOLCHAIN and EXTERNAL_TARGET_SYS based on the
installed toolchains in CODEBENCH_PATH/../toolchains.
This adds a new variable, rather than allowing EXTERNAL_TOOLCHAIN to point to
CodeBench instead of the actual toolchain path, to keep the clean separation
between the generic toolchain logic and that which is specific to our
toolchains, and also avoids overloading the variable's meaning.
JIRA: SB-7881
Signed-off-by: Christopher Larson <chris_larson@mentor.com>
