Set PREFERRED_PROVIDERS in sample local.conf.

Several non-machine specific PREFERRED_PROVIDERS were set in
the machine configuration file.  These really belong in distro
settings or in local.conf if not set.  This commit sets them in
the local.conf.sample so users need to update their local.conf to match.
For new installs, the standard initialization does the right thing:
 $ TEMPLATECONF=meta-gumstix-extras/conf source ./poky/oe-init-build-env