Put proper DMA streams for stm32l DAC in mcuconf.h
fix allowed DMA stream mask for DAC in hal_lld.h
use LEDs available for stm32l-discovery board
Description reflects the STM32 architecture
Put DAC DMA streams definitions into hal_lld.h for STM32L1xx

DAC DMA stream masks check is hard coded for these streams
anyway. So there is no point to keep them in mcuconf.h, since there is
no way to change them without modifying architecture headers.

Probably this should be done for other STM32 architectures.,
The demo readme explains oscilloscope setup.