[NaCl SDK] Use nacl_config.py in common.mk build system.

Also, remove GetLoaderPath, GetHelperPath, GetIrtBinPath from getos.py.

BUG=none
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/118553007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243629 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243629