Update TODO

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
gcc-runtime-external: use GCC_VERSION, not BINV

BINV is e.g. 4.8, we want e.g. 4.8.3.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
*-external-cross: provide non-appended version

That is, also provide binutils-external-cross, not just
binutils-external-cross-arm.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
tcmode: use ??= for gdbserver, oprofile

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
*gcc*external: be consistent with the version variables

Signed-off-by: Christopher Larson <chris_larson@mentor.com>