Projectkk2glider/lcd dma in background

First commit enables DMA transfer of LCD buffer in the background, while the menu task continues. It uses only one lcd buffer so if code starts to write on LCD before the data is sent (by DMA) it could overwrite it. This commit also changes make behaviour - the intermediate .o files are not removed every time - means shorter compile time, but if make options change one must use `make clean` before the make.

The second commit introduces dual LCD buffers (6k more RAM usage), one is used for DMA sending while the other is used for LCD drawing functions. Classical double buffering. 

Third commit tries to call `perMain()` on a more regular basis of 20ms.
Moved out from opentx.h