fix(config): Remove make.conf options that duplicate profile defaults.

We were already using the default for CFLAGS, etc. LDFLAGS had
--as-needed but that has gone into the default Gentoo profile so we
ought to move it there as well. Also, ld's -O1 and -O2 are equivalent so
use the profile default -O1.
fix(config): Move USE_PYTHON into profiles.

Unlikely this is actually needed right now but it isn't hurting either.
Will remove it once we move to Python 2.7.
fix(config): Update the SDK make.conf to refer to the current version.

This means that we will always get the correct set of binary packages
for our current manifest version.
fix(cros_sdk): Update default SDK tarball version.

Update to latest release. cros_sdk is still using this file instead of
version.txt in the manifest because it needs a tarball that already
exists when building a new version for the first time. At some point
that will work differently but for now we'll have to keep this file.
Merge pull request #48 from marineam/updates

Updates and cleanups, use new release for binary packges