Fix bug: VBUS (5V USB) was not connected.

Fix bug: VBUS (5V USB) was not connected.

The error was that in the schematics the VBUS _net_ was (at least I
thought that) connected to the VBUS _pin_ on the mini-USB connector.
That, however, was not the case. The VBUS _net_ is not automatically
connected to the VBUS _pin_ just because they share the same name.

The fix is to explicitly create a VBUS net line on the VBUS pin on the
USB connector (in addition to the +5V pwoer symbol there). That will
then really connect to the VBUS net on the 3.3V regulator on the other
side of the schematics.