USB UART seems to work fine at 115.2Kbps or 230.4Kbps, but starts to drop characters
as the data rate goes higher. This commit changes the usbuart ISR to fill a software
FIFO, and adds a low priority timer interrupt to run deferred processing to drain a
FIFO and send USB CDCACM packets, rather than calling the usb send within the UART
ISR.
Tested on native platform, up to 1.5MBps.
- stm32_mem.py has problems with erasing the big pages, but dfu-util works
- serial GDB remote server doesn't work. It neither works for the STM32F107,
so maybe there is a problem with the usbd_f107_driver.
