more font improvements and cleanup

Get rid of the notion of a font's "ascent"; the reference point for
drawing is the top-left corner of the character box rather than the
baseline.  Add some more space between the menu entries and make the
highlight bar around the text.

Replace the default font.png with two images; the build system will
include one or the other based on the resolutions of the device.

Restore the original compiled-in bitmap font, to fall back on when
font.png can't be found (eg, in the charger binary).

Add support for bold text (when a font.png image is used).

Change-Id: I6d211a486a3636f20208502b1cd2aeae8b9f5b02