CDC implementation without any timers.

[makarenya]

Changed version of #388 pull request, without timers and with reduced count on copy memory operations.

I tried to achieve the following goals: remove the dependency on the timer, and reduce the number of memcpy operations.

For example, the STM32F103C8T6 doesn't have so many timers, and using one of them for the CDC, is not so good.

And memcpy operations.
In fact, they occur too often. First, in the USBSerial::write method the data is copied from the caller buffer into the UserTxBuffer, then the CDC_TIM_PeriodElapsedCallback the same data is copied into the StackTxBuffer, this data is transferred to USBD_CDC_SetTxBuffer without copying, and from where they are copied to PMA. 3 copy operations, and I managed two.

Same situation with the receive.

[fpistm]

I guess this is the same PR than fpistm#1
Build failed due to missing switch protection.

[fpistm]

I will provide soon a rebase of this PR including some fixes (typo, clean,...).
Tested and works fine. Thanks @makarenya very nice implementation.

Edit: Not functional with USB HS.

[fpistm]

About HS not working (Discovery F746NG and Win10):

• Device well recognized
• analogReadSerial test show nothing
• Try to disable I_CACHE and D_CACHE without success
• USB receive test bench result:

port COM14 opened
ignoring startup buffering...
ignoring startup buffering...
ignoring startup buffering...
ignoring startup buffering...
ignoring startup buffering...
Bytes per second = 1
Bytes per second = 3838772
Bytes per second = 1
Bytes per second = 3840000
Bytes per second = 1
Bytes per second = 1
Bytes per second = 3840246
Bytes per second = 1
Bytes per second = 3840000
Bytes per second = 1
Bytes per second = 1
Bytes per second = 3839754
Bytes per second = 1
Bytes per second = 3840737
Bytes per second = 9220839
Average bytes per second = 1

[makarenya]

Closed in favor of #410