Scons: Simplify the code that sets the TRUSTED_ENV key
Scons: Simplify the code that sets the TRUSTED_ENV key

The code was overly general and it was not obvious what it was
setting.  It would also set UNTRUSTED_ENV, but this is not used, so we
have removed this case.

Split SanityCheckAndMapExtraction() into separate functions since it was
doing three unrelated tasks.

BUG=none
TEST=check that run_hello_world_test still runs, but that we get sensible
  warnings with --mode=nacl or --mode=dbg-host

Review URL: http://codereview.chromium.org/6594031

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@4425 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client