gummiboot/builder: fix timeout setting when unset

The gummiboot-builder.py script is expecting the @timeout@ metavar to be
substituted for either an empty string (in the case where a user has
left the timeout unset) or the actual value set in the system
configuration.

However, the config.boot.loader.gummiboot.timeout option defaults to
'null', and due to the way pkgs.substituteAll works, the substitution
for '@timeout@' is _never_ set to the empty string.  This causes the
builder script to put a bogus line into /boot/loader/loader.conf:

   timeout @timeout@

Fix this by explicitly setting 'timeout' to the empty string when it's
unset in the system configuration.

Signed-off-by: Josh Cartwright <joshc@eso.teric.us>