USB: allow to configure the maximum packet size of EP0

USB: allow to configure the maximum packet size of EP0

All USB devices support endpoint 0 (EP0) when powered up. This endpoint
is the target of the default pipe. After the attachment of a device has
been detected, the USB software uses endpoint 0 to initialise the
device, perform generic (i.e., non device-specific) configuration, and
obtain information about the other endpoints provided by the device.

The descriptor of EP0 is hard-coded into the low-level device driver of
each target platform, and cannot be re-defined by the user to implement
a custom USB device. So, expose the maximum packet size of this special
endpoint as a configurable parameter into the wizard.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4699 38d2e660-2303-0410-9eaa-f027e97ec537