sugar: update dependencies

Add missing vte3 and libabiword dependencies, which prevented
Terminal and Write from working on a Sugar-only build.
base: create compat symlink for bootfw too

Spotted by German in Nicaragua, an XO-1 running Q2E45 doesn't
auto-upgrade the firmware when 13.2.0 is installed. This is because
it looks for bootfw.zip whereas the installed file is named bootfw0.zip.

This breaks the rule that post-release commits should not change
the default build output. This one does, because it adds a symlink.
Given the current situation and the simplicity of this fix,
we'll break the rules.
base: don't enable olpc-update timer when it is not installed

olpc-update is not always installed, so we should not try
unconditionally to enable its systemd timer.

Fixes build failure when versioned_fs=0
v7.0.1 release