These boards were broken in commite1d409f Refactor USB pulluphandling. Before that commit, all boards without external controllablepullups were assumed to have fixed external pullups and use the DP writetrick. Since that commit, boards that have internal pullups are assumedto *not* have any external pullups and the internal pullups areautomatically used.It turns out there exist some boards that have internal pullups in thechips, but also have an external fixed pullup. This can probably beconsidered a hardware bug, but since the boards exist, we should supportthem.This commit allows variants to define USBD_FIXED_PULLUP to explicitlystate they have a fixed pullup on the D+ line. This will cause the D+output trick to be applied even when internal pullups are present,fixing these boards.This define is only needed on these specific boards, but it can also bedefined on boards with a fixed external pullup without internal pullups.This problem was prompted by the "Black F407VE" board, which has theproblematic pullup. All other F4 boards were checked and one other wasfound to have the pullup, all others seem ok. None of the other serieshave been checked, so there might still be board broken.See alsoSTM32-base/STM32-base.github.io#26 forsome additional inventarisation of this problem.Thisfixesstm32duino#1029.

While reviewing some of these boards, it was not immediately clear to mewhat boards they referred to (especially with the relatively unbrandedboards from aliexpress). Just in case this helps anyone else, this addssome urls with more info on those boards I found from the git historyand github issues.

This board also has internal pullups, so the external one is notactually needed (and will even violate the USB spec when both are usedtogether). This commit disabled the external pullups, but leaves thedefines in comments, as future documentation.See alsostm32duino#1029.