bootchart: improve output based on Dave Jones' feedback

Dave Jones, in his blog, had some feedback about the bootchart script:
Primarily his complaint was that shorter delays weren't visualized.

The reason for that was that too small delays will have their labels
mixed up in the graph in an unreadable mess.

This patch has a fix for this; for one, it makes the output wider,
so more will fit.
The second part is that smaller delays are now shown with a
much smaller font for the label; while this isn't per se
readable at a 1:1 zoom, at least you can zoom in with most SVG
viewing applications and see what it is you are looking at.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>